Place baby spinach in a large bowl; set aside. In a large skillet cook onion in the 1 teaspoon oil over medium heat until tender and slightly golden, stirring occasionally. Stir in 1 tablespoon of the jalapeno jelly. Add sweet pepper; cook and stir for 1 minute more. Remove from heat. Stir onion mixture into spinach; cover and set aside. Wipe skillet clean.
Meanwhile, rinse fish; pat dry with paper towels. Cut fish into 4 serving-size pieces. Sprinkle fish with the salt and pepper. In the same large skillet heat the 2 tablespoons oil over medium-high heat. Add fish and cook 2 minutes on each side to sear. Reduce heat to medium; cook about 5 minutes more or until fish flakes easily when tested with a fork. Transfer fish to a serving platter; cover and keep warm.
Add the remaining 3 tablespoons jalapeno jelly to skillet. Cook and stir until melted; spoon over fish. Toss spinach mixture with vinegar. Serve spinach mixture with fish. Makes 4 servings.