Whisk together ingredients for white sauce and refrigerate until ready to use.Combine lime juice, oil and cumin in a shallow nonreactive dish small enough to just hold the fish. Add fish and turn to coat in the marinade. Season with salt and pepper and marinate fish at room temperature for 30 minutes. OR mix dry rub spices together and sprinkle on both sides of fish.Preheat grill to medium-hot. Brush grill grate with oil. Grill fish until just cooked through, about 3-5 minutes per side. Break fish into large pieces.Fill tortilla with fish and desired accompaniments. Drizzle with white sauce and squeeze fresh lime wedges over tacos.