Cut the fish fillets into thick strips, grease a baking sheet with oil and place the fillets into this baking sheet. Season them with salt and white pepper. Set aside.
Grate the mozzarella cheese and set aside.
Into a bowl, add the milk, the table cream, the egg and the mustard, mix very well, add the grated cheese and mix again.
Pour this mixture all over the fillets and bake in a preheated oven at 200°C (392°F) for 40 to 50 minutes.
Serve.