Preheat oven to 375 degrees F.
Line meatballs on a parchment lined rimmed cookie sheet. Place in oven and cook for about 20 to 25 minutes.
In a large cast iron pan, or skillet, cook the onions in about 3 to 4 tablespoons olive oil over medium heat. Stir occasionally until onions brown. At medium heat it should take about 15 to 20 minutes. Once the onions are golden brown add the roasted red peppers, and cook while stirring for about 2 to 3 minutes. Remove from heat and drain excess fat if desired.
Slice open the sub buns but not all the way through. Leave them attached, then just set them on a parchment lined cookie sheet cut side up.
Transfer the meatballs to a large skillet and sprinkle taco seasoning over top. Add enchilada sauce to the pan, then cook over medium heat, stirring until the meatballs are coated and the mixture is combined well. Take off heat.
Place about 5 meatballs in each bun. Scoop any extra sauce over top of each sub. Top with onion and pepper mixture. Then add cheese over top.
Place the pan in the preheated 375 degree F oven and cook for about 10 to 15 minutes, just until the cheese melts.
Remove from oven and dig in.
Serve with a big old meatball loving smile!