Preheat the oven to 400 degrees F (200 degrees C).
Rinse and pat fillets dry. Rub fillets with olive oil, and season with garlic salt and black pepper. Place each fillet on a large sheet of aluminum foil. Top with jalapeno slices, and squeeze the juice from the ends of the lemon over fillets. Arrange remaining lemon slices on top of fillets.
Carefully seal all edges of the foil to form enclosed packets. Place packets on a baking sheet.
Bake in the preheated oven until fish flakes easily with a fork, 15 to 20 minutes depending on size of fillets.