In a saucepan, warm the butter over medium-low heat. Add the onions and garlic and sauté slowly, stirring frequently, until the onion bits are quite soft and beginning to turn golden, 12 to 15 minutes. Mix in the sugar and cook for about 2 minutes more to caramelize some of the onion bits. Stir in the remaining ingredients and bring the sauce to a simmer. Cover and cook