First season the haddock pieces with the salt and freshly ground black pepper. Arrange the fillets in a single layer in the ovenproof dish (nice enough to bring to the table).
Mix the grated Cheddar cheese together with the Dijon mustard and the cream and spread carefully over the fish.
Bake in the pre-heated oven for about 20 minutes or until the fish is cooked and the top is golden and bubbly. If the fish is cooked but the top is not yet golden, place under the broiler for just a few moments to color the cheese.