Step 1Heat butter in small skillet over medium-high heat. Add onion and apple. Cook, stirring often, until soft, about 7 minutes. Transfer to large mixing bowl along with sausage, pork, bread crumbs, egg, salsa, milk, salt and pepper to taste. Use 2 large forks, wooden spoon or your hands to mix ingredients thoroughly.
Step 2Transfer mixture to 8 x 4-inch loaf pan and shape into compact loaf. Use your hands to lightly press surface to be sure loaf is compact. (Can be baked immediately or made a day ahead and refrigerated, covered airtight. If refrigerated, let rest at room temperature 30 minutes before baking.)
Step 3Bake uncovered until browned, about 1 hour. Use large spatula to transfer to serving platter. To serve, cut crosswise into half-inch slices. Serve hot.