Meanwhile, in 12-inch nonstick skillet, cook sausage over medium-high heat 7 to 9 minutes or until no longer pink; drain. Reduce heat to medium. Add onion, garlic and pepper flakes; cook 3 to 5 minutes, stirring occasionally, until onions are softened. Add half of the spinach; cook and stir 1 to 2 minutes or until spinach has wilted. Repeat with remaining half of spinach.