Ceviche de Atún con Piña

Name: Ceviche de Atún con Piña
Ingredients: 2 medium garlic cloves; 1⁄4 medium white onion; 2 medium serrano chiles; Juice of 4 medium limes (about 1/2 cup); Juice of 1 large orange (about 1/2 cup); 1⁄4 cup soy sauce; basil leaves; 1 1-in. piece of fresh ginger; 3⁄4 lb. sushi-grade tuna loin, fresh or frozen (thawed and well-dried, if frozen), cut into ¼-in. cubes; 1⁄2 medium red onion, finely chopped (about ¾ cup); Kosher salt and freshly ground black pepper; 8 6-in. tostadas; 8 thin, cored pineapple slices; 1 medium avocado, peeled, pitted, and thinly sliced; 2 tsp. black sesame seeds; Fresh cilantro sprigs, for garnish