Toss the brussels sprouts in the olive oil, salt and pepper, spread them out in a single layer on a baking sheet and roast in a preheated 400F oven until golden brown, about 20-30 minutes, flipping half way through.Meanwhile, cook the bacon in a pan, set it aside and drain all but a tablespoon of the grease.
Add the butter to the pan and let it melt and bubble.
Add the onion and cook until tender, about 5-7 minutes.
Add the garlic and cook until fragrant, about a minute.
Mix in the flour and let cook until it just starts to brown a little.
Mix in the milk, mustard, miso, parmesan, brussels sprouts and bacon, season with salt and pepper and let it simmer until it thickens, about 3-5 minutes