1In 12-inch nonstick skillet, heat oil over medium-high heat. Add onion; cook 2 to 3 minutes, stirring frequently, until softened.
2Stir in water and noodles. Cover; heat to boiling. Boil 4 minutes.
3Stir in mushrooms, tuna and pasta sauce (sauce will be thin). Reduce heat to medium; simmer uncovered 4 to 6 minutes or until mushrooms are tender, sauce has slightly thickened and noodles are tender.
4Remove from heat; let stand 5 minutes. Just before serving, top with croutons.