Place the spaghetti squash in a microwave safe dish, cut side down.  Pour about half an inch of water in the dish, then cover with plastic wrap and microwave until soft, about 10 minutes.  Remove and let cool slightly, then with a fork, scrape up the strings from the squash.

Meanwhile, in a pan, heat the olive oil over medium heat.  Add the onion and cook until tender, 5-8 minutes.  Add the garlic and cook until fragrant, about 60 seconds. Add in the sour cream, cream cheese and half of the parmesan cheese and stir until the cream cheese has melted and the mixture is smooth.  Stir in the spinach and artichoke hearts and cook until the spinach has wilted.  Season to taste with salt and pepper. Pour half of the mixture into each spaghetti squash half and gently pull up on the spaghetti squash strings to combine with the sauce as much as possible.  Top with the shredded mozzarella and the remaining parmesan.  Place under the broiler for 2-3 minutes, or until the cheese is melted and bubbly.