Daniel Acevedo (pictured with his partner, Sarah Wasserman), is head chef at Mildred’s vegetarian restaurant, famous for, among other things, its constantly changing (and always vegan) “burger of the day”.

Mildred’s beetroot burger remains one of the best veggie burgers I’ve ever had, and their kidney bean and olive burger was a close second. I wrote about these elusive burgers last year when I described my quest for the ultimate veggie burger and asked: just what are these burgers made of?

Serious about my quest, I went straight to the source, Daniel himself, and spoke to him about how they make the burgers at Mildred’s. Daniel was kind enough to entertain my burger fancies, and even shared a mouth-watering recipe for their popular beetroot and fennel burgers. Turns out, the secret is in the “Sosmix”.

The burgers at Mildred’s rock my world. Are they your own recipe?

Our burgers have been on the menu about 15 years, and I’ve been in the kitchen about six. We have a standard recipe we follow but we change the vegetables and ingredients to flavour them differently. The burger mix changes every two or three days.

What are the fundamental ingredients that go into a cohesive burger?

It begins with Sosmix, the main binder of our burgers, which is a dehydrated soy mince that you can buy at shops like Whole Foods and some grocery stores. There are a few different brands but they’re all fairly similar.

To this we add vegetables, fresh herbs, dried herbs, tinned tomato or water, and a bit of gram (chickpea) flour to hold it together. The vegetables and herbs vary depending on what flavor we’re after. For example, today I made a red pepper, caper and courgette burger, to which I added parsley, oregano and black pepper.

Finally, we add chopped tinned tomatoes or the equivalent amount of water to bring it all together. The amount depends on how much water the sosmix is going to absorb and the types of vegetables we’re using.

I’ve seen bean burgers on the menu – where do those fit in?

We use the same formula and treat beans like one of the vegetables. But you must remember, cooked beans have lots of liquid so you won’t need to add as much water or tomatoes to the mix.

It seems like there’s an art form to achieving the perfect moisture balance – how do you know when the burgers are just right?

With the tomatoes and water, it’s not something you can really measure – start with a little and add it as you need it. And if you are adding beans or juicy carrots, add less liquid.

What about texture? Do you choose certain ingredients for texture?

We generally choose a theme and base our burger around that. So, if it’s Italian then we’ll add black olive and basil. If it’s Mexican, spices. Asian, coriander. Sometimes we put seeds and nuts in the burger but not too often because we deal with a lot of nut allergies.

Do you have any advice for people who want to make a veggie burger at home?

The main thing is not to rush the process. If you’re using our method, you really need to left the burgers sit 30-40 minutes. Then you need to test the mixture. You want a mixture that you can grab in your hands and form into a burger, but doesn’t leave your hand messy. It should be dry enough to mold into a burger shape, but not so dry it crumbles apart when you cook it. It’s an in between thing. Trial and error. Be patient with it.

Check out Mildred’s blog to read about Daniel’s recipes and food experiment with his comrade Sarah Wasserman. And if you’re in London, book a table at Mildred’s for lunch or dinner. You won’t be disappointed.