Bring a large saucepan of water to a boil. Using a paring knife, cut out the stem end of each tomato, then slice a shallow X in the bottom.

Plunge the tomatoes into the boiling water until their skins loosen, about 30 seconds. Remove them with a slotted spoon and let cool. When cool enough to handle, slip off their skins. Discard the water, but save the saucepan for cooking the jam.

Halve the tomatoes at their equator and gently squeeze out the seeds and juice. Cut the tomatoes into 1/2-inch (1.5-cm) pieces.

Return the tomatoes to the saucepan and stir in the sugar, pepper, and salt. Cook over medium heat, stirring frequently to ensure that the mixture is cooking evenly, until most of the liquid has cooked off or a candy thermometer registers 220°F (105°C), about 15 minutes. If foam occasionally rises to the top, skim it off with a large spoon. Remove from the heat and stir in the lemon juice.

Ladle the jam into clean jars. Cover tightly, let cool, and refrigerate. It will keep in the refrigerator for at least 6 months.