Add meatballs to cast iron pan (or other oven safe pan). Cook for about 1 minute per side (just to brown slightly).
Pour the marinara sauce over the meatballs, cover and simmer until meatballs are heated through (approx 15-20 minutes).
Sprinkle cheese over top of the meatballs. You can continue to cook over low heat until cheese is melted or for faster results (plus a little toasted/bubbly cheese), stick the pan in the oven on broil just until the cheese starts bubbling. That will give you the nice little golden cheese spots too!
Serve warm over pasta, spaghetti squash, roasted broccoli or with garlic bread slices.