Heat a large skillet over medium-high heat. Add turkey, onion, and garlic. Cook and stir in the hot skillet until turkey is no longer pink, about 5 minutes. Add 1 1/2 cups salsa, cumin, salt, black pepper, and chili powder. Stir together.
Preheat the oven to 425 degrees F (220 degrees C).
Whisk milk, polenta, butter, corn flour, egg whites, and baking soda together in a bowl. Pour mixture into a 9x13-inch baking pan. Pour cooked ground turkey mixture over corn bread layer.
Bake casserole in the preheated oven for 10 minutes. Sprinkle 1 1/2 cups shredded Mexican-style Cheddar cheese blend on top. Continue baking until top is bubbling, 5 minutes more. Let cool for 10 minutes.
Top casserole with remaining 1/2 cup shredded Mexican-style cheese and 2 cups salsa.