In a large bowl with a cover, combine the crab legs, olive oil, lemon juice and parsley. Stir to coat the crab legs. Cover and refrigerate overnight.
Put the garlic, beer, water and onion into a large pot. Place the crab legs in a steamer basket and set aside. Discard the marinade. Bring the beer mixture to a rapid boil, then insert the steamer basket. Cover and steam for 10 to 15 minutes, or until crab is cooked through.