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.