Bring a large pot of salted water to a boil. Add brussels sprouts, cook until tender, about 5 minutes. Drain and rinse in cold water until cool. Cut large sprouts in half and set aside. I keep the small sprouts whole.
In the same pot, cook bacon until browned; 4 minutes. Drain on paper towel. Remove excess fat from pot.
In the same pot, add the sliced onion and cook until softened, 3 - 4 minutes.
Meanwhile, preheat oven to 350 degrees. Stir spouts and bacon in with the onion. Stir to mix well. Transfer to a 1-1/2 quart casserole dish.
In a bowl, combine heavy cream, salt, thyme, pepper and nutmeg. Pour over sprouts mixture.
In a small bowl combine the bread crumbs with the parmesan cheese. Sprinkle over the sprouts. Dot with butter.
Bake until lightly browned, about 30 minutes. If baking from room temp, bake for 45 min to one hour.