- Start by combining 1 tablespoon of pickling spice, 2 cups of white vinegar, 1 cup of water, ½ cup of sugar, 2 cloves of garlic (peeled and minced), and ½ teaspoon of red pepper flakes in a saucepan. 
- Bring the mixture to a boil over medium-high heat and stir until the sugar has dissolved. Then reduce the heat to low and let it simmer for 10 minutes. 
- Meanwhile, prepare the deer heart by boiling it in hot water for 10 minutes or until cooked through. Then, cut it into small pieces and then place them in a 1-quart jar with the sliced onions and remaining pickling spice. 
- Once the pickling liquid has simmered for 10 minutes, remove it from the heat and let it cool for 5 minutes. 
- Pour the pickling liquid into the jar with the deer heart and onions, making sure to cover it completely. Add 1 teaspoon of kosher salt and mix everything together carefully. 
- Place a lid on the jar and store it in the refrigerator for at least 3 days before eating. The flavor will continue to develop over time.