In a shallow bowl or baking dish stir together the beer, soy sauce, brown sugar, garlic, chili powder, pepper and onion powder. Sprinkle the flank steak on all sides with salt, add to the marinade. Marinate for at least one hour and up to overnight.
In a food processor add the chimichuri ingredients, process until smooth.
Preheat the grill.
Grill the steak until desired degree of doneness, about 4 minutes per side for medium rare. Allow the steak to rest for 5 minutes.
Rub the bell pepper with olive oil, grill until soften and grill marks appear.
Slice the steak and the bell pepper.
Fill the butter lettuce leaves with steak and bell peppers, spoon on sauce.