Slice a spaghetti squash in half and scoop out the seeds. Put the squash, cut side down, in a glass baking dish in the microwave for a few minutes. Fill the scooped-out part with marinara (jarred or homemade) and mozzarella. Then run the squash, sauce and cheese under the broiler in the oven for a few minutes, until the cheese bubbles. Use a fork to scrape out the squash, which turns into spaghetti-like strands, sauce and cheese.