Bring the water to a boil and slowly add the polenta while whisking constantly. Continue to whisk until the polenta is thick and begins to bubble in the center, about 10 minutes. Remove from the heat and pour into a 13x9 glass baking dish that's been sprayed with cooking spray. Refrigerate until solid, a few hours or overnight.
In a large saute pan coated with cooking spray, add the onions and peppers and saute until they begin to get soft. Add the garlic, cumin, chili powder and salt and stir to combine. Add the black beans and stir to incorporate.
Preheat the oven to 350°. Spoon the black bean mixture on top of the polenta and top with the cheese. Heat in the oven for 10-15 minutes until just warm. Serve with fresh salsa, avocado and sour cream if desired.