Place the ground beef and sausage in a large bowl. Add the bread crumbs, sour cream, grated onion, eggs, cilantro, and Old El Paso Taco Seasoning to the meat. Mix by hand until evenly combined and very smooth. There should be no solid chunks of pork or beef.
Use a 3-tablespoon scoop to portion out 24 meatballs of equal size. Roll the meatballs to pack them tight and remove cracks.
Heat an extra-large saute pan to medium heat. Add the butter. Once melted add the meatballs. Cook the meatballs for 10 minutes, gently rotating to various sides every 2-3 minutes. *If you don't have a large enough pan to fit all the meatballs, either use two pans or work in batches.
Pour the enchilada sauce over the meatballs and continue to simmer for 8-10 minutes. Serve as-is or use in Meatball Subs.
To Assemble the Meatball Subs:
Preheat the oven to 400 degrees F. Cut the sub rolls open. Arrange the rolls on a large rimmed baking sheet lined with parchment paper. Open the rolls and place 3 meatballs in each roll. Cut the cheese slices in half. Then lay 2 halves over the length of the meatball subs. Bake for 3-5 minutes to toast the rolls and melt the cheese. Serve immediately.