Whisk beer, onion, brown sugar, olive oil, red wine vinegar, garlic, and hot sauce together in a large glass or ceramic bowl. Add skirt steak and toss to evenly coat. Cover the bowl with plastic wrap, and marinate in the refrigerator, 6 to 24 hours.
Preheat an outdoor grill for high heat, and lightly oil the grate.
Remove skirt from the marinade, and shake off excess. Discard the remaining marinade.
Cook steak on the preheated grill until reddish-pink and juicy in the center, 4 to 5 minutes per side. An instant-read thermometer inserted into the center should read 130 degrees F (54 degrees C). Transfer meat to a plate to rest, about 10 minutes.