All of Max Halley's sentences end with either a massive, joyful exclamation mark, or a boom of laughter.

The London-based sandwicher is the proprietor of cult restaurant Max's Sandwich Shop - and has now decided to take on the state of the picnic, with his new cookbook Max's Picnic Book, co-authored with Ben Benton.

Don't get him wrong, Halley absolutely loves a picnic, but he reckons the time has come: it "needs a hand. It's become this sort of Sound Of Music, Mary Berry's birthday party, chintzy thing covered in rose-tinted goo, when lunch on a train is a picnic! Lunch in the motorway service station car park is a picnic! Lunch at your desk is a picnic!" he rages jovially. "And these are all opportunities for deliciousness that we too often let pass us by!"

The book itself is a little incongruous, surrealist and goofy at first - it features a meat trifle, a picnic dedicated to the sausage, one picnic menu is 'hosted' by Mary Berry and Hunter S. Thompson, and another by Ringo Starr and Debbie McGee. But as you start reading and Halley's radiant enthusiasm seizes you, it begins to make a whole lot more sense - and more importantly, makes you question the very notion of traditional picnicking.

Halley's love of them comes from a simple place: getting tipsy on trains.

"What nicer thing to do?!" he says, with one of those huge laughs. "Breaking the bread with friends or, quite frankly, with anyone, is a wonderful thing. And doing that by a river under a tree, listening to music, having a lovely time with your shoes off, is a beautiful way to do it."

The problem, Halley says, is when it gets to the food portion of the picnic. "We often accept substandard things just because it's a picnic. We think hummus and pitta is enough - and it's not. There's also carrot sticks in a bag and if you're extremely lucky and really going for it, cucumber sticks in a bag. An innovative approach to hummus does not a picnic make."

We need instead to look at picnics differently. "We need to think about how good a friend of ours the thermos flask really is, and why aren't we putting tinned beef, consomme and supermarket tortellini into a thermos flask and having that at our desk for lunch? I mean, that's picnicking like a boss, instead of it being the meal deal from Boots."

A picnic, he adds, isn't about "bunting and jumpers for goalposts and homemade ginger beer. It's about eating something delicious in any place that is not your dinner table.

"We're told that to do a picnic properly, you have to have all your stuff in a wicker basket. And yet, [we know] they're not fit for purpose. A tote bag is way better for carrying stuff; all your bottles aren't going to fall over; it doesn't jangle in quite as annoying a manner; and a tote bag never ripped anyone's jumper!" Halley continues passionately. "[The picnic's] been sort of co-opted and become something it isn't. It's turned into this Enid Blyton abomination that is just not true."

The real truth is, you can buy a pack of dark chocolate digestives, some ricotta and cherry jam from the supermarket "and you're making mini cheesecakes! How much does any one of those individual things cost? Three quid, total? And you and four people can feast on that in a park!"

Completely by accident, Halley appears to have written the book at just about the best time ever to write a book about picnics - the pandemic has made it practically mandatory, and pretty much the only way to socialise (restrictions allowing). "I wish I'd thought of that before," he jokes. "I would have asked for more money."

The book is a nudge to make those moments sat on park benches a little more exciting. "This is what we need," he says. "It's like a little tiramisu. A little pick-me-up - everything's just so bloody grim, isn't it?"

As someone who also owns a restaurant, he adds, "let me tell you, s*** has never been bleaker." At the time of speaking, it's mid-January and Halley's sandwich shop is very much closed - but he remains characteristically optimistic, acknowledging it's the "weirdest time" but sure "everything will be OK".

After all, as far as he's concerned, the most fun you can have at work is running a restaurant. "Even though it's hard, fundamentally you're giving people a good time for a living, without having to take your clothes off," he says. "That's a really genuinely lovely thing to do."

Now 38, Halley's worked in the industry since he was 22, and his dad's a wine journalist, so good food, good wine and entertaining for him is "the be all and end all". You can feel that on every page, the sheer joy he finds in coming up with ever more inventive ways to create something delectable.

The book is full of hacks, like things to do with a leftover jar of dill pickles, when you've eaten all the pickles, brilliant uses for a Swiss Army Knife, ways to make the most of Thermos flasks (he's obsessed and owns around 20, all in different sizes) and, crucially, 'six wonderful things to mix into mayonnaise'.

"At my sandwich shop, there are two mantras," says Halley seriously. "What will happen when I throw that into the deep-fat fryer? And can you mix that into mayonnaise?" These questions are ones we should all live by.

"It's the ideas I'm so proud of," he says with real feeling. "I believe in exploiting life's opportunities for deliciousness. And being inventive. And I like enjoying myself and having fun and feeling like I've done something really cool, like, 'I'm drinking ice-cold white wine on the train out of my cooler bag'." You can see why his 36th birthday involved fancy dress and a mass McDonald's order that saw all his mates concocting freestyle Maccy D mashups with random condiments, with everyone adding booze miniatures to milkshakes. It sounds like the best birthday ever.

"I really do think cooking is one of life's great providers of joy," he says, and hands down, a Max Halley picnic is a picnic you want to be at.

Max's Picnic Book by Max Halley and Ben Benton, photography by Louise Hagger, is published by Hardie Grant, priced £16.99.