Slice the cucumbers into rounds about an eighth of an inch thick. A mandoline set to its thinnest setting works best here. If you are using a knife, go slowly and aim for even slices so they pickle at the same rate.

Place the cucumber slices and onion in a large bowl. Sprinkle with half a teaspoon of kosher salt and toss to coat. Let them sit for ten minutes. You will see water begin to pool at the bottom of the bowl. Drain this off and gently pat the cucumbers dry with a paper towel. This step keeps your brine from getting diluted.

In a measuring cup or small bowl, stir together the white vinegar, water, sugar, and remaining salt until the sugar and salt fully dissolve. Taste the brine. It should be sharp and slightly sweet. Adjust with a pinch more sugar if you want it a little less tart.

Layer the cucumber slices, onion, and fresh dill into a quart jar or a shallow airtight container. Pour the brine over the top. Press the cucumbers down gently with a spoon so they are mostly submerged. Sprinkle the black pepper over the top.

Seal the jar or container and place it in the refrigerator. Let it sit for at least two hours before serving. If you can wait four hours, the flavor is noticeably more developed. The cucumbers will be slightly translucent and smell bright and herby when they are ready.

Serve cold, straight from the jar. Use tongs or a fork to lift slices out. Spoon a little brine over the cucumbers when you plate them for extra flavor.