Shrimp and Scallop Ceviche Tostada

Name: Shrimp and Scallop Ceviche Tostada
Ingredients: 4 cloves, unpeeled; 1/2pound raw sea scallops, cleaned; 1/2pound raw large shrimp, peeled and deveined; 6ounces (about 2 to 3 medium)fresh tomatillos, husked, rinsed and roughly chopped; 1ripe avocado, cut in half, pitted and flesh scooped from the skin; 1/2 to 1fresh serrano chile, stemmed and roughly chopped; 2tablespoons fresh-squeezed lime juice; 1cup loosely packed, roughly chopped cilantro, plus extra leaves for garnish; Salt; 1/2medium cucumber, peeled if you wish and cut into ½-inch pieces; 12 tostadas, store bought or homemade