Cook pasta according to package directions; drain. Set aside.
Meanwhile, cut 1/4 of onion into thin slices; set aside. Chop remaining onion. Heat large nonstick skillet over medium heat until hot; brown Ground Beef with chopped onion 8 to 10 minutes, breaking beef up into 3/4-inch crumbles.
Stir in cooked pasta, sour cream and salsa. Reduce heat; simmer 5 minutes or until heated through.
Divide spinach evenly among four plates. Spoon beef mixture over spinach. Top with tomatoes, avocado, sliced onion and cilantro.
Nutrition information per serving: 538 Calories; 139.5 kcal Calories from fat; 15.5 g Total Fat; 3.9 g Saturated Fat; 0.1 g Trans Fat; 0.4 g Polyunsaturated Fat; 3.3 g Monounsaturated Fat; 89 mg Cholesterol; 543 mg Sodium; 62 g Total Carbohydrate; 3.3 g Dietary Fiber; 6.8 g Total Sugars; 37 g Protein; 0 g Added Sugars; 143 mg Calcium; 7 mg Iron; 821 mg Potassium; 2 mcg Vitamin D; 0.4 mg Riboflavin; 9.4 mg NE Niacin; 0.4 mg Vitamin B6; 3 mcg Vitamin B12; 287 mg Phosphorus; 6.8 mg Zinc; 24 mcg Selenium; 108.1 mg Choline;