Preheat your oven to 350 degrees with the rack in the middle position. On a parchment lined baking sheet, roast the spaghetti squash whole for 45 minutes-1 hour, until soft when squeezed. Rotate the spaghetti squash half way thru baking. Remove the squash from the oven and allow to cool for 20 minutes.

While the spaghetti squash is cooling, preheat a large skillet over medium high heat. Add the olive oil and cook the sausage meatballs until golden brown, about 8 minutes, stirring occasionally. Add in the onion and zucchini to the meatballs, season with salt and pepper, and continue to cook until the vegetables soften, about 5-8 minutes. Add the garlic and cook for 1 more minute. Add in the marinara sauce, and reduce the heat to low. Simmer for a 5 minutes minutes.

While the sauce is simmering, using a very large knife cut the spaghetti squash in half lengthwise. Scoop out the seeds and discard. Using a fork, scrape out the spaghetti squash noodles and transfer to a plate. Season with just a pinch of salt and pepper. Top the spaghetti squash noodles with the meatballs and marina sauce. Serve warm.