In a large saute pan or skillet, melt 4 tablespoons of the butter over medium-high heat. Add the onions and cook, stirring, until soft, about 3 minutes. Add the garlic and cook until fragrant, about 30 seconds. Add the chicken livers, thyme, salt, and pepper and cook, stirring, until the livers are browned on the outside and still slightly pink on the inside, about 5 minutes. Add the Marsala and cook until most of the liquid is evaporated and the livers are cooked through but still tender.