Bring milk to a medium boil and add your beurre maine or French kneaded butter in the size of peas, stirring briskly until thickened. As soon a sauce is cooked turn off your heat.
Puree all your veggies in saucepan to a cream with a stick blender.
Add white pepper, nutmeg to taste and a pinch salt only if needed.
If you are using a casserole dish you may gently slip your haddock fillets back in under the sauce. Cover surface with grated cheese and place casserole under grill element until cheese has melted and is to your liking.