This is how I have always cooked it.I always eat the heart the day of or the day after the kill; by far the best meat of the whole animal.
I slice it crossways to the chambers to make rings/circles about 1/2" thick or so, roll them in a bit of flour and pan fry in butter.
Doesn't get much better than that! so not really a recipe, but that's what I do with it.
Done veal hearts before in the oven with stuffing of one sort or another, but fresh deer heart is king IMO.