Boil pasta until al dente. Saute onions in olive oil for 5 minutes or until translucent. Add hot pepper, anchovies and whisk in until anchovies dissolve into a paste. Add peppers and celery and saute for 5 minutes or until tender but still somewhat crunchy. Stir in capers and tuna and cook just to warm the tuna through. Season to taste with salt; remove hot pepper; add black pepper if you wish. Drain pasta and return to pasta pot; add sauce and toss; transfer to serving bowl & garnish with parsley.