In a large bowl, using a rubber spatula, mash together the brie, blue cheese and butter. Stir in the chopped onion and paprika.
Stir in the beer, a little at a time, until the cheese mixture is soft and slightly lumpy. Season with salt and pepper. Refrigerate until firm but spreadable, about 1 hour.
Scoop the chilled cheese spread with an ice cream scoop onto small plates, top with some of the sliced onions and dust with paprika.