
Slice the onions into rounds using a sharp chef's knife or a mandoline. You can choose thinner slices for a softer texture or thicker slices for more crunch, depending on your preference. Use your fingers to separate the layers, then transfer the onion slices to a large glass jar. Press firmly to pack the onions tightly inside.
Combine the water, vinegar, sugar, salt and any optional pickling spices in a large bowl, then whisk together until the sugar and salt are completely dissolved.
Pour the pickling liquid over the onions in the jar until the onions are completely submerged. Seal the jar and let it stand at room temperature for 2 hours.
Once the onions are tender and bright pink, they're ready to enjoy. Serve immediately or store in the refrigerator for up to 2 weeks.