Trim the stumps off of the Brussels sprouts off. Place them in a vinegar water bath for at least 20 minutes (filtered water in a large bowl with a splash of vinegar). I use apple cider vinegar but any vinegar will do. Drain.
Blanche the Brussels sprouts (Bring them to a boil in cold water, and boil for about 10 minutes). I use filtered water. Drain.
Sauté in butter and once the sprouts are slightly browned, turn off the heat and sprinkle with lemon juice as they cool. It is best to use a sift under the lemon to catch the seeds.
Prepare the white sauce. Dissolve the flour in a fraction of the milk. It's easiest to use a whisk and the measuring cup itself. Stir continuously on medium-low heat until thickened.
Season: Stir in grated nutmeg add mustard 1, nutmeg, salt and pepper
Pour the sauce over the brussels sprouts and cover with cheese. Bake at 400 for 20-25 mins.