Cut sausage into ¾” cubes. Heat a large non-stick skillet over medium-high heat for 3 minutes. Add sausage and onion; cook stirring frequently for 3-4 minutes or until sausage is lightly browned and onions are tender. Remove sausage and onion from pan; keep warm.
Add olive oil and heat until hot; add potatoes and cook 20-25 minutes until golden brown and tender, stirring occasionally. Reduce heat to medium. Add red pepper flakes and garlic; cook 1 minute stirring constantly.
Add sausage mixture and spinach. Cook, stirring gently, just until spinach wilts and mixture is heated through, about 2 minutes. Add salt and pepper to taste. Turn off heat. Sprinkle mozzarella cheese over top and cover. Let stand 2 minutes or until cheese is melted.