Preheat the oven to 400ºF. Carefully cut the spaghetti squash in half lengthwise, then use a spoon to scoop out the seeds and gooey inside, leaving behind the firm squash in the center. (Save the seeds for roasting later, if you'd like!)
To prepare the filling, drain and rinse the canned lentils, then transfer them to a mixing bowl. Add in the marinara sauce and 1/4 teaspoon of salt, and stir well.
Place the squash, cut-side facing up, on a rimmed baking sheet and season the inside with salt. Divide the filling between the two halves, filling up the center cavity, then top each half with the mozzarella cheese.
Bake at 400ºF for 50 to 55 minutes, or until the squash is fork tender and the cheese is golden and bubbly. Let the squash cool for 5 to 10 minutes, then use a fork to scrape the tender squash into strands, and mix it in with the sauce. Season with additional salt, as needed, and enjoy warm. Leftovers can be stored in an airtight container in the fridge for up to 5 days.
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Notes
Nutrition information is for one stuffed squash half. This information will vary depending on the size of the squash, the type of cheese you use, and the brand of marinara sauce.For a vegan stuffed squash, feel free to omit the cheese or use a vegan cheese, instead.