Clean the deer heart by trimming off the hard fat on top of the heart, along with any other pieces that may be attached to the heart yet. Rinse well under cold water while squeezing the heart to expel any blood left inside. Repeat until the water runs clear.
Bring a large pot of water to a boil over high heat and add one sliced onion, one tablespoon of pickling spice, and the deer heart. Boil for 30 minutes, or until the heart is cooked through.
While the heart is boiling prepare a medium saucepan by adding the vinegar, water, sugar, whole garlic cloves, crushed red pepper flakes, salt, and remaining tablespoon of pickling spice.
Set over medium-high heat on the stovetop and bring to a simmer. Reduce the heat and continue to simmer for approximately ten minutes, or until sugar and salt are well dissolved. Then remove from heat and let cool.
When the heart is done boiling set it on a cutting board to cool enough to handle, then cut into bite-size ¼" slices.
Add remaining sliced raw onion and the boiled heart to a mason jar in alternating layers until full. Then pour in enough brine to fill completely, ensuring pickling spice and garlic clove get added as well.
Place a lid on the jar and set in the refrigerator to pickle for at least 48 hours, although the flavor will continue to improve over the next few days.
Notes
For a cleaner taste the pickling spice can be left out, but I really think the flavor of the clove and allspice in my pickling spice blend really make this pickled heart exceptional.