Prepare the jars and lids:
Wash all jars and lids thoroughly with soap and water and rinse well. Fill your canner with enough water to cover the jars by at least 1 inch and bring to a simmer. Using a pair of canning tongs, lower the jars in gently, tilting them to fill with the hot water. In a small saucepan, keep some water warm but not boiling; place the lids in the water. Have an additional kettle of water on to boil.
Peel and core the tomatoes:
Bring a large pot of water to the boil. Have a large bowl of ice water at the ready. Gently lower the tomatoes into the boiling water and blanch them for 60 seconds. Remove with a strainer and transfer them to the ice water. Once cool, slit the skins; they should peel off easily. Remove the cores with a small paring knife. Discard skins and cores.
Chop the vegetables:
Wearing latex or plastic gloves, chop the jalapenos finely, removing veins and seeds if you wish to reduce the heat. (If you wish to reduce the heat further, replace some with regular green peppers, but do not increase the total weight of peppers in the recipe.) Chop the onion finely. Chop the tomatoes coarsely. Add the peppers, onions, tomatoes, lemon juice and salt to a large saucepan.
Cook the salsa: Bring the mixture to a boil, then reduce the heat and simmer for an additional 10 minutes.
Fill and close the jars: Using canning tongs, remove the jars from the canner, carefully pouring the water back into the canner. Set next to the salsa in the saucepan. Turn the heat under the canner to high. Use a ladle to pour the salsa into the jars through a canning funnel, leaving 1/2-inch headspace at the top. Run a clean chopstick around the inside of the jar to dislodge any trapped air. Wipe the rims of the jars with a damp paper towel. Place the lids on, and screw on the rings until just finger-tight.
Seal the jars:
Using canning tongs, gently transfer the jars to the canner, taking care to keep them vertical. When all the jars are in the canner, there should be at least 1 inch water covering them; if you need more, add water from the kettle until the jars are sufficiently covered. Bring the water to a full rolling boil, and process for 10 minutes.
Remove and cool:
Using canning tongs, gently remove the jars from the canner and transfer them to a kitchen towel or cooling rack, again keeping them vertical. Do not set hot jars directly on to cool counter surfaces. Leave to cool, undisturbed, for at least 12 hours. If any of the jars do not seal when cool, reprocess using the method above, or refrigerate and use immediately.
Label and store:
Add a label to the lid or side of your jar, noting the date it was canned. Remove the rings and store jars in a cool, dark place for up to one year. Refrigerate after opening.