Wash the cucumbers well under cold water. Slice them into thin, even rounds. Peel them if you prefer softer pickles, but the peel adds extra crunch. Slice the onion thinly and place both vegetables into a clean bowl.

Add vinegar, water, sugar, and salt to a saucepan. Stir well over medium heat until the sugar and salt dissolve completely. Add garlic, peppercorns, mustard seeds, dill and red pepper flakes if using. Remove from the heat.

Place the sliced cucumbers and onions into a clean glass jar. Pack them gently without pressing too hard. This helps the brine reach every slice and keeps the cucumbers nice and crunchy.

Carefully pour the warm pickling brine over the cucumbers until everything is covered. Use a spoon to remove any trapped air bubbles. Let the jar cool completely before placing the lid on top.

Place the jar in the refrigerator for at least four hours. Overnight is even better because the flavors become stronger. Serve the pickled cucumbers cold for the best crisp texture and refreshing taste.