Preheat the oven to 400 degrees F (200 degrees C). Line a baking sheet with parchment paper.
Cut the spaghetti squash in half lengthwise. Rub olive oil on the flesh and season with salt and pepper. Place the squash halves, flesh-side down on the baking sheet. Poke the skin a few times with a fork. Bake for 30 minutes.
Meanwhile, add 1/2 tsp of salt to the ground beef and mix well. Form 1 ½” round meatballs. Brown the meatballs in a medium skillet over high heat, about 8 minutes.
Turn the heat to medium and add the tomatoes, tomato paste, garlic, herbs and remaining salt to the skillet. Stir well. Simmer for 15-20 minutes, while the spaghetti squash cooks. If the sauce is too thick, add 1-2 tbsp water to loosen it up.
When the squash is done, remove from the oven. Using a fork, gently scrap the squash loose from the skin and fluff into strands.
Add half the meatballs and sauce to each squash half. Top with mozzarella cheese. Return the squash to the oven and cook for an additional 8-10 minutes, until the tops are melted and bubbly.