Heat a pan over medium heat - melt butter, stir in gradually flour, let it be golden brown. Stir in milk gradually, then raisins, hazelnuts - stir all thae time. Add the mustard, lemon juice and hot pepper sauce. Stir and heat for 1 minute, then remove from the heat and set aside.
Preheat oven to 200°C; season fish fillets with salt, pepper and paprika on both sides. Place each fillet in casserole, arrange banana slices over the fish and finally pour the mustard sauce over all.
Bake for 30-40 minutes. Serve with rice.