Pickled Beef Tostadas with Tomatillo Salsa

Name: Pickled Beef Tostadas with Tomatillo Salsa
Cuisine: Mexican
Ingredients: 4 large tomatillos, husked, coarsely chopped; 1 serrano chile, seeds removed if desired, chopped; 1 avocado, finely chopped; Kosher salt; 1 1-lb. piece beef brisket, trimmed, sliced against the grain ½” thick; Kosher salt and freshly ground black pepper; ½ habanero chile, seeds removed if desired, finely chopped; ½ serrano chile, seeds removed if desired, finely chopped; ½ teaspoon finely grated lemon zest; ¼ cup fresh lemon juice; ½ teaspoon finely grated lime zest; ¼ cup fresh lime juice; 1 tablespoon fish sauce (such as nam pla or nuoc nam); ¼ head green cabbage, thinly sliced (about 3 cups); ½ red onion, thinly sliced; 4 tostada shells; 1 avocado, cut into 1” pieces; 3 tablespoons finely chopped fresh cilantro