Prepare a large pot of boiling water. Gently scald the tomatoes, then peel, core and quarter the tomatoes. Use your hands and a sieve to squeeze out the seeds and juice. Set aside the pulp for the jam.
Place the tomato pulp in a large saucepan and bring to a boil. Lower heat to a simmer, and let it cook for about 10 minutes, stirring often. Place 3 cups of pulp in a separate large saucepan and stir in the sliced lemons and pectin. Bring to a boil, stirring constantly. Stir in the sugar and continue cooking at a boil for about 3 minutes. Remove from heat and skim off any foam from the surface. Continue to stir as it cools for 5 minutes.
To preserve in a waterbath:
Ladle jam into hot jars, one at a time, leaving 1/4-inch headspace. Wipe rims. Center lids on jars. Apply bands and adjust to fingertip tight.
Place filled jars in canner, ensuring jars are covered by 1 to 2 inches of water. Place lid on canner. Bring water to gentle, steady boil.
Process jars for 10 minutes, adjusting for altitude. Turn off heat, remove lid and let jars stand for 5 minutes.
Remove jars and cool. Check lids for seal after 24 hours. Lid should not flex up and down when center is pressed. Clean and store jars according to manufacturer’s instructions.