Place cauliflower florets into a microwaveable bowl with 200ml water. Cover with cling film or a lid and microwave on full power for 5 minutes or until the florets are soft. Drain off the water, add 20ml butter and a pinch of salt and blend until smooth and creamy.
While the cauliflower is cooking, melt 40ml butter and sauté the onion, mushrooms and garlic until the onions are soft and the mushrooms are light brown.
Turn off the heat and add the lemon zest, juice and chopped parsley.
Place the fish fillets on a plate, lightly salt, sprinkle with a few drops of lemon juice and add a knob of butter. Cover with cling wrap and microwave on full power for 2-3 minutes, until the fish becomes translucent, but is still slightly pink at the thicker parts. Pour any liquid that appears into the cauliflower puree. Cut the fish into bite-size pieces.
Lightly combine the onion and mushroom mixture and the fish with the cauliflower, being careful not the break the fish up.
Spoon the mixture into a baking dish or into individual dishes.
Mix the breadcrumbs with the cheese and top the mixture.
Place the pies under a hot grill for about 5 minutes, until they are heated through and golden on top.
Serve with as is, with a fresh tomato sauce or with a green side salad.