Preheat the oven to 400. Prepare two lined baking sheets with parchment paper or a silpat baking sheet.
Slice the spaghetti squash lengthwise, remove seeds + place on baking sheet.
Spray the spaghetti squash with olive oil and place open face up (see photos). Bake in the oven for 20 minutes.
While spaghetti squash is baking, in a bowl, mix together meatball batter.
Roll in ½ inch balls. Bake for 10 minutes
While meatballs are baking, remove spaghetti squash from the oven and shred by pulling at the center with a fork. Remove about half of the squash from the skins.
Once meatballs have cooked, remove them from the baking sheet.
In a bowl, stir together cooked meatballs, shredded spaghetti squash filling and the tomato sauce.
Stuff the spaghetti squash skins with the mixture. Top with mozzarella + put back into the oven for 10-12 minutes.
Optional: For the last 2 minutes turn the oven on Broil/High to get an extra brown, crispy, cheesy top