Preheat the oven to 200ºC/gas 6, or fire up your barbecue.Peel and finely grate the garlic, then pick and chop the parsley leaves. Combine with the softened butter.
Clean the mushrooms and fill them with the butter. Wrap in tin foil and bake in the oven or place on the barbecue for 15 minutes, or till the mushrooms are soft and cooked.
Split the burger buns or ciabatta and lightly cook in a pan with olive oil.
When the mushrooms are ready, unwrap them and pour their juices onto the buns. Smear with mustard and top with a mushroom, a squeeze of lemon juice and some rocket.