Whisk cornmeal, salt and chili powder with 4 1/2 cups water in double boiler. Cook 40 minutes or until thick and stiff stirring 3 to 4 times. Remove from heat.
Preheat to 375°F Heat 2 tbsp oil in saucepan over med. heat. Add onion and cook about 7 minutes or until softened. Stir often. Add the pepper and cook 5 minutes more.
Stir in squash, tomatoes, garlic, coriander, and cumin. Cook 5 minutes, stir occasionally. Stir in 1/2 cup water and salt. Bring to boil. Reduce heat to medium low and simmer, partially covered for 10 to 15 minutes or until squash is tender. Stir in beans and corn and cook 5 minutes or until slightly thickened, stir occasionally.
Coat a baking dish with spray. Spread 2 cups polenta over bottom of dish. Spoon mix inches Smooth remaining polenta over the top.
Score casserole into 6 squares with knife. Brush top with remaining oil. Bake 30 minutes or until headed through and top is lightly browned.