Add tomatillos, pepper, garlic, and onion to blender and process to a smooth consistency. Add cilantro and salt; blend briefly. Taste and adjust seasoning as necessary.
TO SERVE: Warm comal over low heat for 5 minutes. Working one at a time, add cactus paddles to the hot comal and sear 6 minutes per side. Transfer cactus paddles to a clean cutting board and dice.
Melt lard in a small saucepan over low heat. Drizzle 2 teaspoons melted lard onto hot comal and 1 teaspoon inside the well of each huarache, making sure it is evenly coated. Place huarache on comal, well side down, and cook until crispy, about 2 minutes. Turn over and crisp base of huarache, about 2 minutes more. Remove from heat and transfer to a warm platter.
Top each huarache with ¼ cup frijoles de olla, followed by ½ cup diced nopales, a handful of quesillo, and 2 tablespoons salsa de tomatillo. Serve warm.