Preheat oven to 350 degrees. Make a simple white sauce: Melt the butter, gradually stir in the flour, then slowly add the milk, whisking constantly until thickened. Add the chopped onion, bay leaf, ground cloves, and salt and pepper. Place fish in a greased, shallow baking dish. Pour the sauce over the fish and bake until the fish flakes easily and all translucency is gone, about 20 to 40 minutes, depending on the thickness of the fillets.