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Using a sharp knife, cut the fennel into very thin,1⁄4 inch (0.64 cm) slices and set aside. Add 1 cup (240 mL) of white wine vinegar, 1 cup (240 mL) of water, 1⁄4 cup (59 mL) of sugar, and 2 tablespoons (30 mL) of coarse salt. Continue to boil the ingredients until the sugar and salt are completely dissolved. Experiment with adding additional flavors like orange or peppercorn, or find a recipe online for other options. Do not cool the liquid completely. It should still be warm when you begin the next step. Allow the fennel to steep in the liquid for about 20 minutes. Use it immediately, or place it in canning jars to store it for up to 1 week in the refrigerator. Use the pickled fennel as a topping on salads or meat dishes.

Summary:
Cut a fennel bulb into thin slices. Combine pickling ingredients in a small saucepan and boil. Allow the liquid to cool for 5 minutes. Add the fennel.