Peel red onions and slice them into fine rings (using a mandolin slicer).
Add sliced onion rings, garlic clove, bay leaf, and black peppercorns into an 8 oz (250ml) mason jar.
Add apple cider vinegar, water, sugar, and salt in a saucepan and bring to a boil. Once the sugar and salt have dissolved, pour boiling-hot over the onion rings.
Close the jar tightly, allow to cool briefly, and leave to infuse overnight in the refrigerator.
Notes
Stays fresh for about 4 weeks if stored in the refrigerator.Adjust spices according to taste. You can add chili peppers, red pepper flakes, one cinnamon stick, mustard seeds, sliced ginger, herbs, or any flavor of your choice.