1If fish fillets are large, cut into 4 serving pieces.
2Heat oil in 10-inch nonstick skillet over medium heat. Cook onions in oil 2 to 3 minutes, stirring occasionally, until crisp-tender.
3Stir in pesto and lemon juice. Top with fish. Heat to boiling; reduce heat to low. Cover and cook 5 to 10 minutes or until tuna is slightly pink in center.