Lamb Kidney Canapés with Olive and Anchovies

This is a very simple recipe I tried in my attempts to broaden my kidney-eating horizons.  It's very straightforward, plus it's appetizer-sized, so you don't have to worry your guests with a kidney-centric entree.
Just take a few lamb kidneys, which are the size of a large, two-bite meatball, and split them sideways so you can cut out the hard core of fat.  This creates a small pocket in the center of each kidney, which is perfect for filling with a flavorful compound butter.  Who doesn't like butter?  Since these were lamb kidneys, I was in a lamb-mindset and went with a traditional flavoring of minced anchovies, olives, and a little garlic.  But you could go with fresh oregano and garlic, or perhaps sauteed onions and curry powder whipped into the butter.
To give these canapés their base, I roasted mushroom caps separately.  While you could roast the kidneys on top of the mushrooms, I found the kidneys cook much quicker than the mushroom.  Overcooked kidneys are especially unpleasant, so I did two separate roasting batches to maintain control of the kidney's doneness.  Roast the kidneys at 375 until the sides brown and the butter bubbles down into the kidneys, but still keeping just a bit of pink in the middle.  Then top each mushroom with a roasted kidney, which has the added benefit of absorbing all the delicious lamby, buttery juices.