Place unopened spinach packages in microwave; cook at HIGH (100%) setting for 5-7 minutes, or until packages bend easily, rotating and turning over after 3 minutes. Turn spinach into a colander and drain well. Place spinach in an 8" round glass pie plate. Stir in onion, lemon juice, mustard, salt and pepper, then spread spinach mixture evenly over bottom of dish. Arrange snapper fillets over spinach. Dot with butter or margarine and sprinkle with parsley and paprika. Cover with plastic wrap, leaving one corner open to vent. Microwave on MEDIUM (50%) setting for 3-4 minutes. Stir and rearrange spinach around edges of dish. Replace cover and microwave on MEDIUM (50%) setting 3-6 minutes longer, or until fish flakes easily. Serve with hot cooked rice.