Preheat oven to 350° F.
In a heavy skillet, roast green chili peppers until browned, but not burned. Place in cold water and allow to soak for 30 minutes.
Roast garlic cloves in oven for about 15 minutes or until soft. Remove from oven ; cool, peel, and chop. Combine chilies, garlic, oregano, cumin, and beef broth. Process until smooth then strain through a medium sieve.
In a saucepan, heat 1 Tbsp oil. When hot, add the chili puree, reduce heat and simmer, stirring, until thickened, about 15 minutes. Add beef broth and cook for 45 minutes, stirring frequently. Season with salt and pepper. Brush over beef sirloin roast and marinate for 2 hours in refrigerator.
Grill for 20 to 25 minutes, or until meat reaches a temperature 140° F for medium done. Brush often with 1 1/2 tsp oil. Reheat the sauce and serve over the sliced meat.Servings: 8