With a small pairing knife, open the peppers and remove stems and seeds, carefully and without completely flattening the peppers, in order to stuff them. Mix the tuna with the mayo, chopped oregano, chopped basil, half the lemon juice, cayenne, salt, and black pepper. Mix and carefully fill the piquillo peppers. In a separate bowl, toss the rucola lettuce with the pesto, remaining lemon juice, olive oil, and season to taste. Serve peppers on a bed of salad.