Melt the butter in a heavy skillet, then add the onion, scallions, carrots, celery and bell peppers. Cook, stirring often, for about 5 minutes. Then add garlic and cook until the moisture from the vegetables has evaporated, about 5 minutes more. Set aside to cool; then cover and refrigerate until chilled, at least 1 hour.
Preheat the oven to 350 degrees. In a mixing bowl, beat together the eggs, ketchup, half-and-half, cumin, nutmeg, cayenne, salt and pepper. Add the ground beef, sausage and bread crumbs. Then add the chilled vegetables and mix thoroughly with your hands, kneading for 5 minutes.
Form the mixture into a loaf, approximately 12 inches long, 5 inches wide, and 2 1/2 inches high, on a baking sheet. Bake until cooked through (or 160 degrees F on a meat thermometer), about 1 hour. Let rest 20 minutes before slicing.
Note: The passive work time also includes the 1 hour chilling time for the vegetables. The vegetables may be prepared ahead of time, in the morning or night before, chilled, and used later in order to save time when actually making the meatloaf.