Prep the veg - Peel and thinly slice the cucumber in rounds. If you're feeling fancy, you could scrape the tines of a fork down the sides of the cucumber before you slice it. Peel and quarter the onion, then slice it as thinly as you can. Give a handful of dill a rough chop.
Make the brine - In a medium bowl, mix the water, vinegar, salt, and sugar together. Whisk until the salt and sugar are completely dissolved. Add the cucumbers, onions and dill to the bowl, and stir to combine.
Let the flavors meld - Stash the bowl in the fridge for at least 30 minutes.