Add 3 Tbsp. oil to large freezer-weight resealable plastic bag. Place fish in single layer in bag; turn to evenly coat both sides of each fish piece with oil. Place 2 lemon slices on top of each fish piece. Partially seal bag, leaving small opening at top of bag. While holding top of bag, slowly lower bag into sous vide, pressing top of bag to completely seal bag after excess air has been removed. (Bag should be completely submerged in water.) Cook 20 min. Carefully remove bag from water.