Mix the balsamic, 2 teaspoons of oil and rose harissa. Use to baste the mushroom and put to one side for 20 minutes or so
Gently fry the red onion in a teaspoon of olive oil till caramelised. Put to one side and keep warm
Grill the mushroom on a griddle or barbecue, top side up for 2-3 minutes, then turn. Baste well with whatever is left of the marinade
Continue to grill, topping the mushroom with the cheese slice for the last 2 minutes. Put to one side and keep warm.
Split the burger bun, grill the inside till toasted.
Put your burger stack together. Top the base of the roll with the mushroom (cheese up), then add a good spoon of caramelised onion, then add tomato, then salad greens. If liked, add mustard, ketchup, mayo or a mixture of all three.