When the waiter at Jimmy O'Neill's brings the tavern's signature dessert to your table, the natural reaction is to sit back for a moment of awe. It resembles a triangular brick, three inches high and smothered in black Belgian chocolate that's giving off steam. At its side is a mountain of whipped cream, made fresh that day. It takes a few bites to get the full effect. The cake is very sweet, but the dark chocolate takes the edge off and adds just a hint of bitterness. Then you notice, faintly, the beets. Each slice has four ounces of them, which gives the center a warm, earthy taste. One slice for $4.55 is too big and too sweet to be handled by a mere mortal. Better to enjoy with a friend.