Slice the white and light green parts of the leeks into circles and wash. Pat dry.
In a saucepan, combine the white wine vinegar, water, salt, sugar, garlic cloves, peppercorns, and fennel seeds. Heat over medium heat, stirring occasionally, until the salt and sugar dissolve. Remove from heat.
Stuff the sliced leeks into a 10-ounce or larger jar with lid. Pour the vinegar mixture over the leeks. It should just barely cover them, but don't worry, they will shrink down while pickling. Seal the jar tightly.
Let the jar cool to room temperature, then put the jar in the refrigerator. You can now eat the pickles whenever, but they will have a stronger flavor if you let them sit in the fridge for a day or two. Enjoy within two weeks.
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Notes
Add pickled leeks to salads, sandwiches, toast, charcuterie boards, and pizza.
Feel free to add other seasonings, such as fresh herbs like dill, thyme, or rosemary, whole caraway or mustard seeds, or red pepper flakes.
Please note this is a quick pickle recipe and is not intended to be suitable for canning or shelf-stable pickling.