Thaw fish, if frozen. Rinse fish; pat dry with paper towels. Cut fish into four serving-size pieces, if necessary. Set aside. Place spinach in a large bowl; set aside. In a large skillet cook onion in 1 tablespoon of the oil over medium heat for 5 to 6 minutes or until tender and slightly golden. Add sweet pepper; cook and stir for 1 minute more. Remove from heat. Stir onion mixture into spinach; transfer to a serving platter. Set aside.
Meanwhile, sprinkle fish fillets with salt and black pepper. In same large skillet heat remaining 2 tablespoons oil over medium-high heat. Add fish; cook for 4 minutes. Carefully turn fish. Reduce heat to medium; cook 3 minutes more or until fish begins to flake when tested with a fork. Place fish fillets on top of wilted spinach; cover to keep warm.
In a small bowl stir together the balsamic vinegar and honey. Add to skillet. Cook and stir until heated through, scraping up any browned bits. To serve, spoon balsamic vinaigrette over fish and spinach.