Double Smash Burger on Brioche

Finely chop a quarter of the onion and transfer to a small bowl along with the white wine vinegar and sugar. Mix well to combine and set aside, stirring occasionally.

Slice the mushrooms and cut the rest of the onion into half-rings. Heat a drizzle of olive oil in a frying pan over high heat and fry the onion and mushrooms for 4 - 5 minutes. Stir in the butter and season to taste with salt and pepper, then fry for another 4 - 5 minutes. Remove from the pan and set aside.

Shape the mince into two balls per person and transfer to the same pan. Use a spatula to press down into burger patties of 1cm thickness. Fry for 2 minutes per side over medium-high heat (see Tip). Top with the grated cheese, then stack the patties so as to make a double burger. Reduce the heat and cover with the lid so as to allow the cheese to melt.

Tip: if you're cooking for more than two people, use an extra pan or fry the burgers in batches.

Slice the tomato and set aside. Combine the smoky tomato ketchup with the finely chopped onion and half of the mayonnaise. Season to taste with salt and pepper.

Did you know... mushrooms are high in phosphorus, which works with calcium to strengthen bones and teeth. Phosphorus is mainly found in meat, fish and dairy, but along with mushrooms there are also other plant-based sources such as pulses and wholegrains.