First season the haddock pieces with the salt and freshly ground black pepper. Arrange the fillets in a single layer in the ovenproof dish (it should be a dish that is posh 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.
The fish can be prepared ahead and refrigerated at this point. It will keep in the fridge for a day.
To cook, place 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 a hot grill for just a few moments to colour the cheese.