In a large non-stick skillet over medium heat, soften the onion and jalapeno in the oil. Deglaze with the lime juice. Add the fish and cook 2 to 3 minutes per side, depending on the thickness of the fillets, until cooked through. Season with salt and pepper. Break the fish into large pieces with a fork. Add the remaining ingredients and adjust the seasoning.
Spoon the fish mixture into the taco shells. Serve with sour cream.