Preheat oven to 350 degrees. Grease an 8-inch loaf pan. In a skillet over medium high heat, add butter and sauté the onion until caramelized, about 10 minutes. Add the garlic and cook for another 2 minutes to soften. Set aside to cool, or add onion to a bowl and refrigerate to chill. In a large bowl, soak breadcrumbs in the milk for 5 minutes. Add the chilled onion mixture, beef, sausage, eggs, onion, sage, salt and pepper, mixing well to combine. Place in greased loaf pan, patting mixture to form a loaf. Make the sauce: In a small bowl, combine the brown sugar, catsup, nutmeg and dry mustard, mixing well. Spread sauce over top of meatloaf. Bake for 1 hour or until cooked through. Serves 8