4 slices thick-cut bacon, cut into pieces
1 small yellow onion, diced
3 cloves garlic, minced
2 tablespoons all-purpose flour
1 cup milk
2 tablespoons dijon mustard
1 tablespoon balsamic vinegar
Kosher salt and black pepper
1 cup Parmesan cheese, grated
1 cup Panko breadcrumbs
4 tablespoons unsalted butter
1 tablespoon thyme, chopped + 2 sprigs for garnish
Preheat oven to 350°F. Coat a 9x13-inch casserole dish with cooking spray. Set aside. Blanch Brussels sprouts in a pot of boiling water until just tender. Drain and transfer to a bowl of ice water to stop the cooking process. Cook bacon in a large pan. Transfer to a paper towel-lined plate to drain. Remove most of the excess grease from the pan. Add 2 tablespoons butter to the same pan. Sauté onions and garlic until fragrant. Whisk in flour and simmer until just golden brown. Add mustard, milk, balsamic vinegar, 1/2 teaspoon kosher salt, ½ teaspoon black pepper, and ½ cup Parmesan cheese. Stir until sauce begins to thicken. Mix Brussels sprouts in sauce then transfer to prepared baking dish. In a separate pan on medium heat, melt 2 tablespoons butter with 1 tablespoon thyme. Add panko breadcrumbs and ½ cup Parmesan cheese. Toss until just combined. Mix in bacon pieces. Sprinkle bacon pieces and breadcrumbs over top of Brussels sprouts. Transfer dish to oven and bake 15-20 minutes until sauce is bubbly and breadcrumbs are golden in color. Cool 5 minutes before serving. Garnish with fresh sprigs of thyme.