Meanwhile, cook and stir onions in large nonstick skillet on medium heat 3 to 4 min. or until softened. Add zucchini and mushrooms; cook 3 to 4 min. or until zucchini is softened, stirring frequently. Add half the spinach; cook 2 to 3 min. or just until wilted. Stir. Repeat with remaining spinach. Remove from heat.