In a food processor bowl or food grinder, process or grind the ham and the onion until finely chopped. Set aside.
In a large mixing bowl, stir together the beaten eggs and milk. Mix in the bread crumbs, mustard, and pepper. Add the ham mixture and uncooked sausage; mix well. Pat meat mixture into a 9x5x3-inch loaf pan.
Bake in a 350 degree F oven for 40 to 50 minutes or until done (160 degree F). Let stand for 10 minutes. Transfer to a platter.
Prepare Mustard Sauce and serve over ham loaf.
In a small saucepan, stir together sugar, dry mustard, and cornstarch. Gradually stir in half-and-half, light cream or milk. Cook and stir until thickened and bubbly; cook and stir for 2 minutes more. Gradually stir about half of the mixture into beaten egg yolk. Pour egg yolk mixture into remaining hot sauce in saucepan. Stir in vinegar. Bring to a gentle boil. Cook and stir 2 minutes more.