I recently made beef heart bourguignon and it was delicious. There many recipes on internet; just omit the flour and it is still fine. I did something like this.
1) cube and brown meat (I used tallow)
2) cover in dutch oven and braise in wine and stock on very low heat until tender
3) cooked bacon in separate pan, then added that
4) cooked mushrooms and onions in bacon grease, added those
5) thyme, bay, salt
6) some parsley sauce for serving
Beef heart is a little tougher so I like it slow cooked this way or ground up and mixed with regular beef for chilis and other ground meat concoctions. You can marinate and cook it fast, but hearts of smaller animals are more tender and turn out better this way.