Salt and pepper the steak on all sides, place in a resealable plastic bag. Whisk together the lemon juice, garlic, beer, cummin, chili powder, brown sugar, soy and Worcestershire sauce, pour over the steak, seal the bag well.
Refrigerate for 4-6 hours. Remove from marinade and pat dry.
Grill on a preheated grill until medium rare, about 6-8 minutes per side. Remove from grill, allow to rest for 5 minutes before slicing.
To make the sauce add the remaining ingredients to a blender or food processor, process on high until smooth. Serve steak topped with avocado sauce.