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Field mushrooms are probably the nicest and easiest to use. Clean them – Italian Lucy told me never to wash mushrooms or they absorb the water and become tasteless – by brushing any soil off, and if they’re not quite at their freshest, peeling them. Put a bit (half teaspoon?) of olive oil in each one and swirl it round or rub it round gently. Sprinkle some salt and freshly ground black pepper in each. Place them in a lightly oiled oven tray. (At this point, nowadays, I put these in the oven for a short pre-cook before filling them. But you don't need to, unless you have a very quick-cooking topping, or if you just want to grill rather than bake, the topping). 

Fill each mushroom up high with a (previously made) mixture of fresh breadcrumbs, chopped parsley, chopped garlic, salt and black pepper and bingo! grill or bake until the top is crisp and almost crusty. For an extra flourish you could even put parmesan on top. And for further variants try filling the mushroom with breadcrumbs and blue cheese (as in the picture). Works!