You can add more flavor to this dish a few different ways.  First, add more fat to the dish. You can do this by switching from milk to a heavy cream. This will also make it more creamy.  Second, this recipe is typically a pretty basic dish, but there's no reason you can't add spices and herbs to suite your own taste. Try some paprika, cayenne or red pepper flakes for spice, or sautéing some leeks with the onion.