Thaw fish, if frozen. Pat dry with paper towels. Season with salt and pepper. Slice one lemon; halve other lemon. In a medium bowl toss bread crumbs with tarragon and a pinch salt. In a very large skillet heat 1 Tbsp. butter over medium heat. Add crumb mixture. Cook and stir 4 to 5 minutes or until toasted. Remove from skillet.
In the same skillet heat 2 Tbsp. butter and the olive oil over medium heat. Add onions and a pinch of salt. Cook and stir 10 minutes or until tender but not brown. Add lemon slices and spinach; toss until spinach is lightly wilted. Remove mixture from skillet.
Heat remaining 1 Tbsp. butter in the skillet over medium heat. Add fish. Cook 4 to 6 minutes or until fish flakes easily with a fork, turning once. Transfer to four plates; top with crumb mixture. Serve with spinach mixture. Squeeze lemon halves over each serving.