Cook pasta according to package directions. Drain and return to pan.
Meanwhile, heat oil in large skillet over medium heat. Add onion and garlic; cook 1 to 2 minutes until onion becomes translucent, stirring constantly (watch carefully so garlic does not burn). Stir in tomatoes with juice, parsley, lemon juice and capers. Cook 1 minute; add red pepper flakes to taste.
Reduce heat to medium-low; cook 3 to 5 minutes to thicken sauce, stirring frequently. Gently stir in tuna; heat through.