Preheat the oven to 400°F.
Cut the spaghetti squash in half lengthwise, scoop out the seeds with a spoon, and place the halves cut side up on a rimmed baking sheet. Bake for 45 to 60 minutes, until the strands of the squash easily pull away from the shells with a fork.
While the squash is cooking, make the meatballs: add the ground beef, onion, garlic, and seasonings to a large bowl, then work the mixture with your hands until the ingredients are fully incorporated.
Line a rimmed baking sheet with parchment paper, then form the meatballs into 1-inch balls and place on the baking sheet. Bake at 400 F for 18 minutes. Note: you can bake the meatballs at the same time as the spaghetti squash, just make sure to place the meatballs on the rack above the squash.
While the meatballs bake, add the olive oil in a pot over medium heat. Once hot, add the onion and garlic and sauté for 3 to 4 minutes, until the onion is translucent and the garlic is fragrant.
Add the remaining ingredients to the pot and stir to combine. Bring to a boil to cook the alcohol off the wine, then reduce the heat to medium-low and cook, stirring occasionally, for 5 to 7 minutes, until the sauce is bubbling and slightly reduced.
To assemble the boats: remove the spaghetti squash from its shell and toss the strands with the sauce, then scoop the spaghetti squash back into the shells on a rimmed baking sheet. Place the meatballs on top of the spaghetti squash, then sprinkle with the mozzarella cheese. Return to the oven to bake at 400 F for an additional 10 minutes, until the cheese is browned and bubbling.
Garnish with fresh parsley and serve!