In a large skillet, over medium-high heat, melt the butter and bacon grease. Add the brussels sprouts to the skillet and give the pan a quick toss. Try to have as many brussels sprouts cut side down as possible.
Leave the brussels sprouts for 4-5 minutes, allowing the bottoms to caramelize and then toss to allow them to rotate. Add the bacon, red onions, salt, and pepper and cook for an additional 4-5 minutes or until the brussels sprouts are tender with golden surfaces. Add garlic and sauté until fragrant, about 1-2 minutes.
Transfer the brussels sprouts to a dish and serve while hot.
When you cook your bacon, reserve the bacon grease in a mug for this recipe! Just cook your bacon as usual and then carefully pour the liquid grease into a mug until ready to measure and use.
Do not add the garlic too early, most people accidentally burn their garlic because they add it in too soon to their recipes!