Preheat oven to 500 degrees F.
Heat a large skillet over medium high heat. Add remaining 2 tablespoons extra-virgin olive oil, corn, garlic and chiles. Saute 3 minutes, add zucchini and yellow squash and onions, season with salt and pepper, chili powder; cook 7 to 8 minutes. Add stewed tomatoes and heat through. Transfer to baking dish. Cut 1 tube of polenta in 1/2-inch slices lengthwise. Top vegetables with polenta and cheese. Place in hot oven to melt cheese and warm polenta, 8 to 10 minutes. Garnish with chopped scallions and cilantro or flat-leaf parsley.