Preheat oven to 375°.
Place the nopales and chiles on a baking sheet coated with cooking spray; bake at 375° for 20 minutes or until chiles are blackened and nopales are tender, turning nopales once. Place chiles in a zip-top plastic bag; seal. Let stand 15 minutes. Peel chiles; cut in half lengthwise. Discard seeds and membranes; chop chiles to measure 1/2 cup. Chop nopales to measure 1 cup.
Reduce oven temperature to 350°.
While nopales and chiles cook, prepare the rice. Heat oil in a Dutch oven over medium heat. Add onion and garlic, and sauté 5 minutes or until tender. Add rice, and sauté 2 minutes. Add broth, and bring to a boil. Cover, reduce heat, and simmer 25 minutes or until liquid is absorbed. Cool slightly.
Add nopalitos (chopped nopales), chiles, yogurt, sour cream, and pimiento to rice mixture; stir until well-blended. Spoon mixture into an 11 x 7-inch baking dish coated with cooking spray, and sprinkle with cheese. Bake at 350° for 12 minutes. Broil 8 minutes or until lightly browned.