Preheat the oven to 400°
In a large cast iron skillet, cook the diced bacon until crispy and remove it from the pan. Let it drain on a paper towel.
Add the brussels sprouts, chopped onion and garlic to the skillet and cook them in the bacon grease over medium high for about 5 minutes or until the sprouts are slightly charred and begin to soften.
Season the brussels sprouts with salt, pepper, garlic powder and thyme and stir everything together.
Stir in the heavy cream, cooked bacon and half of the Gruyère cheese. Reduce the heat to low and let it simmer for 2 minutes.
In a small bowl, stir together the breadcrumbs, Parmesan cheese, lemon zest, the rest of the Gruyere cheese and a little salt and pepper. Sprinkle the breadcrumb mixture over the brussels sprouts.
Place them into the oven for 15-20 minutes or until the breadcrumbs become toasted.
Finally, let them sit for 10 minutes, serve warm and enjoy.