1. In a medium saucepan, combine apple cider vinegar, water, sugar, salt, whole peppercorns, mustard seeds, garlic, and bay leaf. Bring the mixture to a boil, then reduce the heat and let it simmer for 5 minutes.
2. Add the sliced beets to the saucepan and cook for another 5-7 minutes until the beets are tender but still have a slight crunch.
3. Remove the saucepan from the heat and let the pickled beets cool to room temperature.
4. Once cooled, transfer the pickled beets and the pickling liquid into a glass jar or container.
5. Cover the jar and refrigerate the pickled beets for at least 24 hours to allow the flavors to develop.
6. Serve the pickled beets as a side dish, on top of salads, in sandwiches, or enjoy them on their own as a tasty snack.