Bring water to a boil, let it sit for a minute, then pour it over the onion slices.
Let them sit for 10 seconds, then drain and rinse with cold water.
Add vinegar, lemon juice, water, sugar, and salt to the pan.
Stir until sugar and salt dissolve, then bring to a quick boil.
Place onion slices in a clean jar.
Add thyme, then pour the hot brine over the onions.
Seal the jar immediately.
After just 20 minutes, your onions are ready to eat.
For the best flavor, wait 3–4 hours.
Store in the fridge, and they will keep for at least 4 weeks.