Mushroom burgers with mustard mayo and pickles

SERVES 4 FOR THE BURGERS Preheat the oven to 210C/190C fan/ gas 6½. Have ready 8 portobello mushrooms and 30g unsalted butter. Trim the mushroom stalks level with the cups and arrange them in a large roasting pan cup-side up. Dot with the butter and season. Halve 350g cherry tomatoes, peel and finely chop 2 banana shallots. Toss these together with 2 tbsp olive oil and some seasoning and pile the mixture into the mushrooms. Scatter over 40g pine nuts. Roast for 25-35 minutes until golden on top. TO SERVE Slit and toast 4 burger buns or oven-bottom muffins. Place a mushroom on each half and serve 2 per person with any juices drizzled over and mustard mayonnaise (for example Maille brand), cocktail gherkins, caperberries and watercress on the side.