Preheat a barbecue or griddle pan. Cut the Burger Buns in half, then toast, cut-side down; set aside.
Mix the lemon zest and juice with the thyme, the 3 tablespoons of olive oil, garlic and seasoning to make a marinade.
Griddle or barbecue the mushrooms over a moderate heat, stalk-side down for 5-7 minutes, then flip over and spoon over the Butter and continue to cook for 5-7 minutes or until tender. Transfer to a plate and keep warm.
Drizzle the rest of the olive oil over the halloumi. Return your griddle pan to the heat and, when piping hot, add the slices of halloumi and cook for about 1 minute each side until dark golden griddle marks appear.
Spread the base of each roll with chilli Jam. Add a couple of slices of halloumi, a garlic mushroom and a pile of salad leaves to each one and serve.