Place sliced cucumber, onion, peppercorns, and mustard seeds in a glass jar.
In a medium-sized saucepan, bring the vinegar, water, salt, and sugar to a boil.
Lower the heat to a simmer and stir until the salt and brown sugar dissolves completely.
Gently pour the hot liquid into the jar over the cucumber and onion mixture.
Make sure that all the cucumber and onion slices are submerged poking around if you need to.
CAREFULLY tap the jar or bowl to remove any trapped air bubbles.
Cool, cover, and store in the refrigerator until ready to add to your favorite food.
Notes
Slice as thin as you can. A mandoline set to 1/8 inch is ideal. The thinner the slice, the faster the brine penetrates and the crunchier the final pickle.
Let the brine cool slightly before pouring if your jar is cold from the fridge. Pouring boiling liquid into a cold glass jar can cause it to crack.
Taste them at 4 hours, then again the next morning. The flavor deepens significantly overnight and you may find you prefer them after a full 24 hours.
If you want garlic dill pickles, add the garlic and dill to the jar raw. The hot brine will mellow the garlic without making it taste cooked.
Keep a jar going all summer. Once you finish a batch, you can start a new one the same day. They are that fast to put together.