Warm 3 tablespoons of butter in a small skillet set over medium heat. Add onions and cook until soft, stirring frequently (about 4 minutes). Add the garlic and thyme and cook, stirring constantly, for 1 minute more. Take the pan off the heat, and stir in the remaining 5 tablespoons of butter. Once the butter is melted and incorporated, season to taste with kosher salt and freshly ground black pepper.