Preheat the oven to 425°F. Carefully poke the spaghetti squash several times with a knife, making sure to cut through the flesh and into the cavity inside. Microwave the whole, poked squash for 4 minutes.
Once the squash is cool enough to handle, remove it from the microwave and cut it in half lengthwise. Scoop out any seeds. Pro-tip: these can be cleaned and roasted like pumpkin seeds!
Place the squash halves cut side down on an oiled baking sheet or jelly roll pan. Bake for about 30 minutes, or until the squash is cooked through ( depending on the size of the squash, this may take as many as 45 minutes.
While the squash is cooking, set a large skillet over medium heat. Add 1 Tablespoon of olive oil to the hot pan. Add the garlic and the spinach and cook until the spinach wilts, 2-3 minutes. Stir in the heavy cream, pesto, and the parmesan. Turn off the heat. Add salt and pepper to taste.
Remove the squash from the oven and lower the temperature of the oven to 375°F.
Using a fork, scrape the inside of the squash to loosen the strands of "spaghetti." Split the spinach/cream mixture between both spaghetti boats, and stir each one to combine the mixture with the strands of squash.
Top each spaghetti boat with 3 ounces of mozzarella and bake at 375°F for 20 minutes.After 20 minutes, turn the broiler on and broil for 2 minutes to brown the cheese.