Preheat oven to 375-400º F.
Slice polenta into one-inch rounds, about seven slices.
Warm olive oil in a large skillet over medium heat. Add sliced polenta.
Brown each side of the sliced polenta, heating about five minutes on each side.
Remove the polenta from the pan and place temporarily on a plate. If you used an oven-safe pan for heating the polenta, you may reuse it for the oven bake portion of this recipe as well.
Layer butterbeans, mushrooms and onions in the pan. Add half of the marinara sauce.
Blend the sauce with the vegetables.
Layer the polenta over the vegetables, and then add the remaining marinara sauce on top of each slice.
Sprinkle mozzarella cheese over the marinara sauce, then layer on parmesan to top the dish off.
Place the pan in the oven, uncovered, for 20-25 minutes until the cheese is melted and the dish is bubbling.
Remove from oven, let cool for a few minutes, then serve over a plate of arugula.
Garnish with basil (or oregano), then serve.