Heat a large skillet over medium heat and spray with cooking spray. Add onions; cook and stir until golden, 5 to 7 minutes. Add soy crumbles and mushrooms; cook until browned and softened, about 5 minutes. Stir in tomato sauce, oregano, garlic powder, salt, and pepper. Reduce heat to low; cook until sauce is heated through, about 2 minutes.
Spray a skillet with cooking spray. Cook polenta over medium heat until golden, about 4 minutes per side.
Place 2 pieces of polenta on each serving plate; top with sauce.