Sterilize the jar by washing it in hot, soapy water, rinsing well and drying it in the oven for 10 minutes at 300 degrees F.
Mix salt, sugar, and coriander seeds in a bowl.
Thinly slice the lemons.
Scatter a small amount of the salt and sugar mixture on the bottom.
Take the lemon slices, dip one side in the mixture, layer them with the salty side down in the jar, and gently press down to extract juice as you continue. Repeat dipping, layering, and pressing until you’ve reached near the top.
Add the remaining salt sugar mixture. Make sure the lemon slices are all covered with juice. You might need to squeeze juice from extra lemons to top off the jar.
Seal the jar and store in a cool, dark place for 1 week, turning the jar upside down each day to let the salt and juice redistribute.
After a week, store the jar in the fridge. Wait at least 3 weeks before opening it.
Preserved lemons will keep in the fridge, submerged in juice for a year. Be sure to keep the lemon slices submerged in the salty lemon juice or mold will grow.
To cook with the lemon slices, remove them from the jar and rinse with cool water.