Preheat oven to 350°.
Drain and rinse grape leaves; set aside.
Divide ginger, garlic slices, and lime slices evenly. Place in cavity of each fish; set aside.
Cover a baking sheet or jelly-roll pan with rock salt to a depth of 1/2 inch. Arrange half of grape leaves on top of salt, leaving a 1 1/2-inch border. Place fish on top of leaves. Cover the fish with 10 leaves, tucking leaves around fish. Place remaining leaves loosely over fish. Cover fish and leaves with a 1/2-inch layer of rock salt. Bake at 350° for 35 minutes. Remove fish from oven; let stand 10 minutes.
Combine lime juice and next 4 ingredients (lime juice through serranos). Set aside.
Cover a large work surface with newspaper. Tip baking sheet gently over newspaper to remove loose salt. Use a small spatula to gently remove fish, still in grape leaves. Remove any salt from outside of leaves; peel away leaves, and discard. Arrange fish on individual plates; serve with sauce. Sprinkle fish with cilantro.
Note: You can use this technique with most any fish, including fillets and steaks. If you use these cuts, place the ginger, lime slices, and garlic slices on top of the fish.