In a medium bowl, combine the Kefir, mayonnaise, lime juice, cilantro, cumin, salt, and pepper until well incorporated. Store in the refrigerator for up to 1 week.
For the Tacos:
Preheat a grill to medium-high. Place the fish in a medium dish. In a separate bowl, whisk together the oil, lime juice, cumin, jalapeño, and cilantro. Pour over the fish, and let marinate for 15 to 20 minutes.
Lightly oil the grill grates, remove the fish from the marinade, and place onto the hot grill, flesh side down. Grill the fish for 4 minutes on the first side. Gently flip and cook for an additional 30 seconds to 1 minute, or until the fish is nearly opaque and flakes apart easily. Let rest for 5 minutes, then flake the fish with a fork.
Place the tortillas on the grill and grill for 20 seconds. divide the fish among the tortillas and garnish with shredded lettuce and the cilantro sauce. Serve immediately.