Melt butter in a heavy skillet. Add onions, scallions, carrots, celery, bell peppers and garlic. Cook, stirring often, until the moisture from the veggies has evaporated, about 10 minutes. Set aside to cool; then cover and refrigerate until chilled, at least one hour.
In a mixing bowl, beat together the eggs, ketchup, milk, cumin, nutmeg, cayenne, salt and pepper. Add the ground turkey, 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 x 5 x 2 1/2 inches, on a baking sheet. Bake until cooked through (or 160 degrees F on a meat thermometer), about 1 hour. Let rest 20 minutres before slicing.