Preheat oven to 375° F.
Melt the butter in a heavy skillet, and add the onion, scallions, carrots, celery, bell peppers, and garlic. Cook until the moisture from the vegetables has evaporated, about 10 minutes. Set aside to cool; then refrigerate, covered, until chilled, about 1 hour.
Combine the salt, black pepper, white pepper, cayenne, cumin, nutmeg, and eggs in a mixing bowl, and beat well. Add the ketchup and half-and-half. Blend thoroughly.
Add the ground beef, sausage, and bread crumbs to the egg mixture. Then add the chilled vegetables and mix thoroughly with your hands, kneading for 5 minutes.
With damp hands to keep the mixture from sticking, form the mixture into an oval approximately 17 x 4 1/2 x 1 1/2 inches.
Place the meat in a baking dish, and place the dish inside a larger pan. Pour boiling water into the larger pan until it reaches halfway up the sides of the baking dish.
Place the pan in the oven and bake for 35 to 40 minutes.
Remove the baking dish from the water bath, and let the meat loaf rest for 20 minutes before slicing and serving.