Preheat your oven to 425° F.
In a bowl, mix together the milk, spinach, sun-dried tomatoes, and mozzarella. Season with chili flakes (if desired), salt, and pepper.
Place the halved spaghetti squash on a baking sheet and season the cut sides with salt and pepper. Sprinkle about 1/4 cup of parmesan cheese into each squash half before dividing the milk mixture among them. Cover with foil and bake for about 15 minutes.
Combine ground chicken (or your chosen protein), Italian seasoning, dijon mustard, and remaining parmesan in another bowl. Season with salt and pepper and mix well. Roll into tablespoon-sized meatballs—around 15 to 16 should do!
Once your squash has baked for its initial time, remove it from the oven and take off the foil. Arrange your meatballs around each squash half on the same baking sheet. Bake again for another 15 minutes until the meatballs are fully cooked.
Remove the baked squash from the oven. Sprinkle provolone cheese over each half of squash before placing meatballs on top. Bake again for about 10–15 minutes until everything is melted and bubbly.
Chop up roasted garlic cloves and mix them with butter and fresh herbs. Spread this garlic herb butter over your meatballs right before serving. Use a fork to scrape out strands from your spaghetti squash while mixing in the melted cheese.