Preheat the oven to 180C/350F.
Brush the smooth underside of the mushrooms with olive oil and place on a baking tray with the gills facing up.
Mix the Pesto with the olive oil and divide this between the mushrooms (aprox 1 tablespoon of pesto for each mushroom), and spread this into the cavity.
Bake for 15 minutes then remove and place the cheese over each mushroom.
Return them to the oven and bake for a further 5 mintes until the cheese has melted.
While the mushrooms are roasting, brush the burger buns with butter and toast on a griddle pan until golden brown
Assemble the burger with fresh rocket leaves, slices of red onion or any other garnish of your choice