In a large skillet, cook the bacon over medium-high heat until crispy.
Remove the bacon from the pan, leaving the grease.
Add the brussels sprouts and cook until slightly softened, about 5 minutes.
Add the garlic and salt and cook another minute.
Chop the cooked bacon and toss it with the brussels sprouts, along with 1 cup of Gruyere cheese.
Transfer the mixture to a greased 9x13” baking dish.
Drizzle with the heavy cream and then top with remaining cheese.
In a small bowl, stir together the melted butter and breadcrumbs.
Sprinkle the breadcrumbs over the gratin.
Note: at this point, you can cover with plastic wrap and refrigerate until ready to bake, up to one day in advance.
Bake the gratin in the preheated oven for 20-25 minutes, until the breadcrumbs are golden brown. If you are baking this after being in the refrigerator, expect to add about 5-10 minutes to the baking time.
*I quartered half of the brussels sprouts and shredded the other half