First, preheat the oven to 375.
Lightly spray an 8×8” square dish (or similar) with cooking spray.
Pour the tomato sauce in the bottom of the dish and swirl to coat it evenly.
In a large bowl, combine all ingredients except the Swiss cheese.
Mix everything together very well, but also try not to over-mix and make the meatballs tough.
Using a 1-tablespoon measure, scoop out 1 tablespoon of the meat mixture. Press a cheese cube into the center of the meat mixture, and top it with another tablespoon of the meat mixture. Use your hands to completely seal the cheese inside the meat, and roll a few times in your hands to seal the edges.
Repeat with all remaining meat mixture and cheese cubes, and place them in the prepared baking dish.
Bake for 18 minutes. If the meatballs are coming from the fridge or freezer, tack on an extra 3-4 minutes of baking time.
Let cool for 5 minutes before serving.