Rinse the pepper under cold water. Cut in half, remove the seeds and inner white, and then cut into bite-sized strips or cubes.
In a large skillet over medium heat, saute the onion in the olive oil until translucent. Add the peppers, and cook for about 10 minutes until lightly golden.
Drain the tuna, and add to the skillet. Add salt and pepper to taste.
Meanwhile, add the pasta to the boiling water. Cook until al dente and drain, retaining some of the cooking water. Add the pasta to the skillet and mix well, adding some of the cooking water if it seems dry.