Tostadas con Nopales y Queso Panela

Name: Tostadas con Nopales y Queso Panela
Ingredients: 2 cups chopped cooked fresh nopales or rinsed jarred or canned nopales (see Note); 3/4 cup chopped fresh cilantro; 1 medium plum tomato , chopped; 1/2 cup chopped white onion; 2 tablespoons extra-virgin olive oil; 2-3 tablespoons lime juice; 1/8-1/4 teaspoon salt plus 1/4 teaspoon, divided; Ground pepper to taste; 9 medium tostada shells , preferably baked (see Note); 2 ripe medium avocados; 3 tablespoons cubed queso panela or crumbled queso fresco