Cut off the ends of the squash and slice lengthwise. Scoop out the seeds and inner parts of squash with a spoon. Discard.
Heat oil in a skillet and add chopped onion. Cook until translucent. Add the garlic and cook for a 30 seconds.
Next, add black beans, tomato sauce, crumbled polenta and spices. Cook until tomato sauce cooks down and the mixture becomes a thick consistency, about 5 minutes.
Salt the inside of the squash, then scoop mixture into squash halves. Line a baking dish with parchment paper. Place squash in the lined baking dish. Bake at 350°F for 20 minutes, or until you can easily slide a knife into it.
Notes
These squash boats will last up to 5 days in an airtight container in the fridge.