Cook the onions in butter in a small saucepan over medium-low heat until tender, about 10 minutes. Stir in the salt and chile powder. Add the beer and simmer for 5 minutes. Remove from heat and cool thoroughly. Place the roasts in a large plastic bag and add the cooled marinade, turning to coat. Close the bag securely and marinate in the refrigerator for 6 to 8 hours (or overnight) turning occasionally.
Remove the roasts from marinade; reserve marinade. Boil the marinade in a pot for 10 minutes. Place the roasts on the grill over medium coals. Grill for 30 to 35 minutes, turning and brushing frequently with the marinade. Roasts will be rare to medium in doneness.
Remove the roasts from grill, tent with aluminum foil, and allow to stand 15 to 20 minutes before carving. Carve across the grain into thin slices. Season with salt and pepper to taste.
