Melt the butter in a large pot over medium-high heat. Stir in the onion, and cook until transparent, about 5 minutes. Mix in the salt and shrimp. Pour the beer over the shrimp, and simmer just until the shrimp turn pink. Turn off the heat, and allow the shrimp to sit at least 1/2 hour. Serve while still warm, or refrigerate and reheat before serving.