Purée in a food processor or blender the garlic, cumin, chipotle peppers, red wine and apple cider vinegars, smoked paprika, and olive oil.
Set aside in a bowl.
Here in the pic are 2 chunks of beef heart. Trim them of all fat, silver skin, and any veins or arteries.
Once trimmed, you will end up with 2 nice pieces of meat.
Cut the meat into 1 1/2” cubes. Add the heart to the marinade and marinate for at least 8 hours, preferably overnight.
When ready to cook, turn on your oven’s broiler, moving the top rack 6” away. Skewer up the heart with pieces of red onion in between the meat. Place on a broiler pan or on a baking rack.
Broil for 5 minutes a side.