In a small (8-inch) dry skillet, roast the garlic over medium heat, turning regularly until they’re soft and blackened in spots, 10 to 15 minutes.
While the garlic is roasting heat a medium-large (4- to 6-quart) heavy pot of water over medium high. When the water comes to a boil, drop in the scallops, reduce the heat to medium-low and simmer the scallops until just barely cooked through (still transluscent in the center), about 3 minutes. Using a slotted spoon, plunge the scallops to a bowl of ice water for a couple of minutes to quickly stop the cooking.
Raise the heat and bring the water to a boil again. Drop in the shrimp, reduce the heat and simmer until the shrimp is just barely cooked through, about 2 minutes. Plunge the shrimp into a bowl of ice water for a few minutes to stop the cooking. Drain the seafood well and cut into ½ inch pieces and scoop into a large mixing bowl.
Peel and roughly chop the garlic and scoop it in a food processor or blender along with the tomatillos, half of the avocado, serrano, lime juice, cilantro and ½ teaspoon salt. Blend until smooth and pour over the seafood.
Cut the remaining half of the avocado into ½-inch pieces and add to the mixture along with the diced cucumber. Stir gently to combine well.
To serve, spoon about ¼ cup of the mixture onto tostadas and garnish with fresh cilantro leaves.