To peel the tomatoes, bring a large pot of water to a rolling boil and prepare a bowl of ice water.

Cut an X in each tomato, then blanch them in boiling water for about 60 seconds until the skins crack and loosen.

Use a slotted spoon to transfer them to the ice water. Work in batches, and repeat for all tomatoes. Let the tomatoes cool briefly.

Once the tomatoes are cool, peel, core, quarter them, and add to your large pot. Set aside.

For the zucchini, trim the ends, then cube or slice it. If the zucchini is large, cut it in half and scoop out the seeds before slicing. Set aside.

Peel and slice onions (if using) into 1/4-inch slices. You'll need 2 slices per jar. Set aside.

Place the jar rack into the pressure canner, and fill it with water per your canner manufacturer's instructions: Presto is 3 quarts, Mirro is 2 quarts, and All American is 2 to 3 inches.

Fill the jars halfway with hot water, then place them on the rack in the canner.

Bring the canner to a simmer for 10 minutes (180˚F). Keep them warm until you are ready to fill them.

Put the pot of prepared tomatoes on the stove, and set it to medium-high heat. Let it come to a boil, then lower the heat to a simmer and cook for about 10 minutes.

Add the prepared zucchini, salt, basil, oregano, and thyme. Stir everything together and let it gently boil for an additional 5 minutes. Keep warm while filling your jars.

Cover your counter with a towel. Use the jar lifter to take one jar out of the canner. Empty out the water (you can save it for washing dishes) and set the jar on the towel. Keep the rest of the jars inside the canner so they stay warm.

Position the canning funnel on the jar, and use the slotted spoon to pack the jar with the hot zucchini and tomato mixture, leaving about 2 inches at the top of the jar.

Remove the funnel and run the bubble popper through the jar to release any trapped air bubbles. Adjust the headspace if necessary.

Wipe off any residue from the jar rim with a damp towel. Place a lid on top of the jar, position the band over it, and gently screw it on until fingertip tight. Place the filled jar back into the canner and repeat the process for the remaining jars.

Place the lid on the canner and lock it. Leave the vent open, adjust the heat to medium-high, and bring the canner to a boil. Allow the pressure canner to vent steam for 10 minutes, place the pressure regulator on top of the air vent, and let the pressure rise.

Adjust the pressure for your altitude if necessary (see chart below), and regulate the heat as needed to maintain a steady pressure.

When the processing time is complete, turn off the heat, and allow the canner to stand undisturbed until pressure drops to 0. Once the canner is depressurized, let the canner cool additional 10 minutes before removing the lid.

Spread a dry towel on the counter, unlock the cover, and remove it by tilting the lid away from you so that steam does not burn your face.

Allow another 10 minutes for the jars to adjust to the change in pressure. If the jars are still boiling vigorously, let them sit in the canner for another 5 minutes or until the boiling slows.

Use the jar lifter to lift jars carefully from the canner and place them on the towel. Keep the jars upright, and don't tighten bands or check the seals yet. Let the jars sit undisturbed for 12 to 24 hours to cool.

After 12 to 24 hours, check to be sure the jar lids have sealed by pushing on the center. The cover should not pop up. The jar failed to seal if the top flexes up and down. Refrigerate it and use it within a few days.

Remove the screw-on bands and wash the jars in warm soapy water. Label, date, and store the jars in a cool, dark place for 12 to 18 months. Once the jar is open, refrigerate it and use it up within a week. Yields about 6 pints or 3 quart jars.