Peel and devein shrimp; set aside.
Coarsely chop 1 mango. Slice remaining mango into 1/4-inch-thick slices; set aside. Combine chopped mango, lime juice, and next 5 ingredients in container of an electric blender or food processor. Cover and process until smooth, stopping once to scrape down sides; set aside.
Coat a large nonstick skillet with cooking spray; add 1 teaspoon olive oil, and place over medium-high heat until hot. Add shrimp, and saute 3 minutes. Remove from skillet, and set aside. Add tuna to skillet, and cook 4 minutes on each side or until fish flakes easily when tested with a fork. Remove from heat, and let cool completely. Flake tuna into large pieces with a fork.
To serve, place 1 1/2 cups spinach on each of 4 plates. Arrange sliced mango, shrimp, and tuna evenly over spinach; sprinkle with onion. Drizzle mango mixture evenly over salads. Serve with breadsticks.