"Beef heart is a dense muscle that takes well to a good acidic marinade. This simple marinade tenderizes and really complements the flavorful meat. Add the smoky heat of a charcoal grill and you've got another arrow in your recipe quiver."
Soak skewers in a bowl or water for at least 30 minutes. Drain water.
Trim the tough outer layer from the beef heart on a work surface using a fillet knife. Cut beef into 1/2-inch wide strips.
Whisk mustard, steak sauce, olive oil, balsamic vinegar, and rosemary together in a bowl and pour into a resealable plastic bag (such as Ziploc(R)). Add beef skewers, coat with the marinade, squeeze out excess air, and seal the bag. Marinate in the refrigerator for 2 to 4 hours, turning occasionally.
Preheat grill for medium heat and lightly oil the grate. Remove beef from marinade and discard marinade. Thread beef onto skewers.
Cook beef skewers on the preheated grill until well browned and firm to the touch, 4 to 5 minutes per side.