Green tomatoes are simply red tomatoes that have not ripened fully. If you are unable to find green tomatoes, you can use Kirby cucumbers.
Toss tomatoes and onion with the salt in a colander set over a bowl. Refrigerate 3 hours. Rinse well, and drain.
Stir together vinegar, sugar, peppercorns, coriander seeds, celery seed, turmeric, bay leaf, and lemon slices in a medium saucepan. Bring to a boil. Stir in tomatoes and onion. Return to a boil. Remove from heat. Let stand until cool, stirring occasionally. Refrigerate in airtight containers overnight or up to 1 month.