Position broiling pan, or rack if using electric oven, 3 inches from the heat, and turn broiler to high.
Spray polenta slices with cooking spray, and place on broiling pan or baking sheet. Broil 5 minutes per side, or until browned.
Meanwhile, heat oil in large skillet over med-high heat. Add onion, and cook 4 minutes or until softened. Stir in lime juice,mexican seasoning, garlic, and lime zest. Cook 4 minutes or until onion is lightly browned.
Add kidney beans and tomatoes; simmer 5 minutes, or until heated through.
Place 3 polenta slices on each plate, ladle bean mixture over top and sprinkle with cheese and cilantro. Serve with sour cream or yogurt, if desired.