Heat olive oil in a cast-iron skillet over medium heat until the oil shimmers, and cook garlic, stirring often, until golden brown and fragrant, about 3 minutes. Stir in onion and cook until soft, stirring often, about 5 minutes. Add Brussels sprouts to the skillet and cook without stirring until the undersides of the sprouts turn golden brown, 5 to 7 minutes. Stir to turn sprouts over, and cook other sides until golden brown, another 5 to 7 minutes. Season with salt, black pepper, and garlic powder.
