Bring a pot of water to a simmer. Add all four whole squash. Cook on low heat until crisp tender, about 8 minutes. Drain the water and let the squash cool.
After squash have cooled, cut off ends and slice lengthwise. Scoop out the seeds and inner parts of squash with a spoon. Chop up the centers.
Heat oil in a skillet and cook chopped onion until translucent. Add the chopped squash and garlic and cook for a minute.
Next, add black beans, tomato sauce, crumbled polenta and spices. Cook until tomato sauce cooks down and the mixture becomes a thick consistency.
Scoop mixture into squash halves. Bake in an oiled pan or baking sheet at 350°F for 20 minutes.
Notes
These squash boats will last up to 5 days in an airtight container in the fridge.