Pack your sliced cucumbers, onion slices, and dill sprigs into a clean, pint-sized Mason jar. Leave about a 1/2 inch of space at the top of the jar for the brining liquid.
In a small pot, heat the vinegar, water, garlic, mustard seed, peppercorns, red pepper flakes, sugar and salt. Allow to simmer gently for about 10 minutes to allow sugar and salt to dissolve and spices to infuse the vinegar.
Allow the brine to cool, but still be warm. Then, fill the jar enough to cover everything with brine.
Close the lid tightly and refrigerate for 24 hours before eating.