Step 1

Bring a pot of water to a boil over high heat. Score an X in the bottom of each tomato. Working in batches, drop them into the boiling water to loosen the skins. When they are cool enough to handle, peel and discard the skins.

Step 2

Place a flexible cutting board inside a rimmed baking sheet; this will help corral the tomatoes and their juices. Chop the peeled tomatoes into small pieces (including the gel and seeds) and transfer to a large colander seated inside a mixing bowl. Press on the tomato pulp to extract as much juice as possible; reserve the juice, if desired, for another use. There should be about 8 cups of pulp.

Step 3

Transfer the pulp to a large pot along with the seeded lemon slices. Cook (uncovered) over medium-low heat for 45 minutes, stirring occasionally; the lemon slices should be quite soft. Stir in the sugar, making sure it has dissolved. Cook uncovered for 15 minutes; the preserves may be a little loose.

Step 4

Cool completely, then transfer to zip-top bags for freezing, pressing to extract as much air as possible before sealing.