Heat the oven to 400º F.
In a large skillet, cook the bacon over medium-high heat until crispy.
Transfer the bacon to paper towels to drain, leaving the grease.
Add the Brussels sprouts to the pan and cook until slightly softened, about 5 minutes.
Add the garlic and salt and cook for another minute.
Chop up the cooked bacon and toss it with the Brussels sprouts.
Add 1 cup of the Gruyère cheese.
Transfer the mixture to a greased baking dish measuring 9 x 13 inches.
Drizzle the mixture with the heavy cream and sprinkle the remaining cheese over the top.
In a small bowl, stir together the melted butter and breadcrumbs.
Sprinkle the breadcrumbs over the gratin. Note: at this point you can cover the gratin with plastic wrap and refrigerate it until ready to bake, up to one day in advance.
Bake the gratin in the heated oven until the breadcrumbs are golden brown, 20-25 minutes. If you’re baking this after it has been in the refrigerator, expect to add about 5 minutes to the baking time.
*I quartered half of the brussels sprouts and shredded the other half.