Preheat the oven to 200°C/180°C fan. Slice the sweet potatoes (skins on) into fries. Add the fries to a large baking tray with a drizzle of vegetable oil and a generous pinch of salt. Toss the fries in the oil until they are fully coated. Bake for 30 min or until golden and crisp.

Meanwhile, remove the stalks from the mushrooms. Place the mushrooms cap side up on a second baking tray (see step photo). Drizzle with oil and sprinkle with a pinch of salt. Bake for 10 min.

Meanwhile, slice the tomatoes. Very finely chop the chives. Grate the mature cheddar.

After 10 min, turn the mushrooms over. Stuff the mushrooms with the grated cheese. Bake for 10 min further, until the cheese is melted and golden.

Meanwhile, in a bowl, combine the mayonnaise, ketchup, chopped chives and Dijon mustard with a pinch of salt - this is your burger sauce.

Meanwhile, place the burger buns in the hot oven for 3-4 min or until warmed through. Load the warmed burger buns with the burger sauce, lettuce leaves, sliced tomatoes, cheesy mushrooms and crispy onions. Serve the fries to the side.