Follow directions in "Canning ABCs: Get Ready" (below), using 7 widemouthed quart jars, rings, and lids.
Meanwhile, peel tomatoes: Fill a large saucepan three-quarters full of water and bring to a boil over high heat. Cook 1 layer of tomatoes at a time in water, just until skins split or will peel easily with a knife, 20 to 40 seconds. Let cool, then core, pull off skins, and trim any brown areas, working over a bowl to catch juices.
Put 2 tbsp. lemon juice or 1/2 tsp. citric acid in each jar. Add 1 tsp. salt if you like. Follow directions in "Canning ABCs: Fill and Seal Jars": Cut tomatoes to fit through jar openings, if needed, and push them into jars to fill compactly, leaving 1/2 in. headspace. Pushing will create juices; if needed, add more juices from bowl so tomatoes are covered. Using the handle of a fork, poke 1 thyme sprig down the side of each jar if you like. Release air, wipe rims, and seal with lids and rings as directed.
Process as directed in "Canning ABCs: Process Jars," boiling for 1 hour and 25 minutes (add 5 minutes for every 3,000 ft. in altitude above sea level). It's okay if jars leak a little. Turn off heat and let the jars stand in water in canner for 5 minutes. Cool, check seals, and store as directed (up to 1 year).
*Buy citric acid in your grocery's baking aisle.
Gather equipment: canning jars with matching metal lids and rings, a boiling-water canner with rack, a widemouthed funnel, tongs, and a jar lifter. Most hardware stores carry these basics.
Fill canner with water and heat it up. The canner should be two-thirds full for pint and half-pint jars; half-full for quart jars. Set rack on pan rim and cover pan. Over high heat, bring water to a boil (180° to 185° for pickles); this takes 30 to 45 minutes.
Meanwhile, wash canning jars and rings in a dishwasher and hand-wash lids; drain. For jam only, sterilize the washed jars too: When water in canner boils, place jars on rack, lower into water, and boil for 10 minutes. Reduce heat to a simmer and keep jars in water until needed.
Nest lids inside rings in a saucepan and cover with water. Heat until small bubbles form (do not boil). Remove pan from heat and cover.
Rinse produce, then prepare as recipe directs.
FILL AND SEAL JARS:
Ladle foods into jars through a wide funnel or arrange with fingers, leaving the headspace (the distance between jar rim and food) specified by the recipe. If the last jar isn't completely full, let cool, then serve or chill; do not process.
Release air bubbles in chunky mixtures: Gently run a knife around inside of jars. Wipe jar rims and threads with a clean, damp cloth so that lids will seal.
Center lids on jars so the sealing compound on lids touches jar rims. Screw metal rings on firmly, but don't force.
Lower jars on rack into water. The water should cover jars by at least 1 in.; add hot water as needed during processing. Cover canner and return water to a boil. Cook for time specified in recipe.
Lift rack with jars onto edge of canner, using tongs and a hot pad. Using jar lifter, transfer jars to towels on a work surface. Don't tighten rings. Cool completely at room temperature. You may hear a "ping" as jars form a seal.
Press on the center of each lid. If it stays down, the jar is sealed. If it pops up, it isn't (you can still eat the food—chill it as if it were leftovers). Label jars and store in a cool, dark place up to 1 year.
Note: Nutritional analysis is per 1/2-cup serving.