About 3 1/2 pounds tomatoes, peeled, cored, seeded, and diced
1/4 cup lemon juice
1/2 teaspoon finely grated lemon zest
1/4 cup torn fresh basil leaves
4 cups sugar
1 (1.75-ounce) packet powdered pectin such as Sure-Jell
Sterilize 5 half-pint jars (in boiling water for at least 10 minutes). Simmer the tomatoes, covered, in a heavy 6-quart pot for 10 minutes. Measure 3 1/2 cups tomatoes and juices and return them to the pot. Add the lemon juice, zest, and basil. Combine 1/4 cup of the sugar with the pectin and stir it into the tomato mixture. Bring to a full rolling boil over high heat, stirring constantly. Immediately stir in the remaining sugar. Return to a full rolling boil that cannot be stirred down, then boil hard, stirring constantly, for 1 minute. Skim off the foam with a metal spoon. Ladle hot into hot jars, leaving 1/4 inch headspace; adjust the lids and process in a boiling-water bath for 5 minutes. Makes 5 half-pint jars.
The lemon zest really helps brighten the flavor, but my mom's original recipe doesn't call for it. Hers uses less sugar—3 cups—and low-sugar pectin, which when I tried it yielded a pretty aggressively jellied jam; it's also processed longer, for 15 minutes, but according to a very similar recipe in So Easy to Preserve a jam with the sugar–lemon juice–tomato proportions above should need only 5 minutes.