Preheat the oven to 400°F (200°C). Toss the quartered brussels sprouts with olive oil, salt, and pepper. Spread them in a single layer on a baking sheet and roast for 20-30 minutes, flipping halfway through, until golden brown.
While the brussels sprouts roast, cook the bacon in a pan until crispy. Remove the bacon and drain all but one tablespoon of the bacon grease.
Add the butter to the pan with the bacon grease. Let it melt and bubble. Add the diced onion and sauté for 5-7 minutes until tender. Stir in the garlic and cook for about a minute, until fragrant.
Stir in the flour and cook for about a minute until it just begins to brown. Gradually whisk in the milk, followed by the mustard, miso (if using), and parmigiano reggiano cheese. Let the mixture simmer for 3-5 minutes, until thickened.
Add the roasted brussels sprouts and crispy bacon to the pan with the mustard cream sauce. Stir to combine, taste, and adjust seasoning with salt and pepper.
Serve warm, garnished with extra parmesan if desired, for a rich and flavorful side dish.
