Preheat oven to 350°, pan spray a 13 x 9 inch baking pan Fry bacon until crisp, remove from pan. Sauté onion 2 -3 minutes in bacon drippings, add garlic. Sauté 1 more minute. Add onion-garlic mixture to baking pan. Top with thawed, halved brussel sprouts.
Melt margarine in a sauce pan. Add flour and cook 3-5 minutes. Whisk in stock, whisk in milk. Add Parmesan cheese, mustard and seasonings.
Pour sauce over Brussels, sprinkle with bacon.
Cover and bake 30 minutes.
Sprinkle with cheese, melt cheese in oven and serve!