Place two sheets of paper towels on a large dinner plate and set aside.

Trim the Brussels sprouts and cut in half. Arrange the sprouts on the lined baking sheet.

Drizzle with olive oil and season with salt and black pepper. Stir with rubber spatula to distribute the oil.

Roast the Brussels sprouts until the oil is sizzling and the outsides are slightly softened, about 10 minutes. When done, remove the baking sheet from the oven.

Reduce the oven temperature to 375°F.

Set a large skillet over medium-high heat. Arrange the bacon strips in the skillet. Fry the bacon until brown and crisp, about 6-8 minutes, flipping once. Transfer the bacon to the paper towel-lined dinner plate.

Finely mince the garlic.

In a large pot over medium heat, melt the butter. Add the garlic and sauté until fragrant, about 30 seconds. Sprinkle the flour into the pot. Whisk to thoroughly combine. Continue whisking until the roux turns slightly golden and begins to smell toasty, 1-2 minutes.

Slowly stream the milk into the pot while continuing to whisk. Cook the mixture, whisking steadily, until it begins to thicken and bubble, 4-5 minutes.

Once bubbling, turn off the heat. Add the Gruyere cheese, salt, and black pepper to the pot. Whisk slowly until the cheese is fully melted and the sauce is smooth.

Once the sauce is smooth, add the Brussels sprouts to the pot. Stir until the sprouts are thoroughly coated.

Pour the contents of the pot into the greased baking dish.

Place the breadcrumbs, Parmesan, salt, and black pepper in a medium mixing bowl. Crumble the cooked bacon into the bowl. Mix with a fork to combine. Add melted butter to the bowl, and stir until all the breadcrumbs are moistened. Sprinkle the crumbs in an even layer over the Brussels sprouts.

Bake the gratin on middle rack of oven until the edges are bubbling and the top is golden brown, 22-25 minutes.

Check to see that gratin is done. Remove from oven or add time as needed.

Allow gratin to rest for 10 minutes. Plate and serve.