Not so sure about the spleen, but I can definitely recommend udder. When I was growing up Belgian butchers sold thinly sliced boiled udder as a cold cut. We'd have it in sandwiches, with a little bit of salt sprinkled on top. I remember it having quite a subtle flavor, and not at all the unpleasant consistency I associate with many organ meats; I definitely loved it as a child. I've never seen it sold in the US though, and even most Belgian butchers have stopped selling it.