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" 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.

Prepare the jam:

Reserve 1/4 cup of sugar in a small bowl; add the pectin and whisk to combine. Set aside. Combine the strawberries and remaining sugar in a large, nonreactive pot; enamel or stainless steel are best (do not use copper or aluminum pots with this method). Let stand for at least 20 minutes, up to two hours or even overnight, refrigerated.

Cook the jam:

Stir the contents of the pot well, and put over medium-high heat. Add the lemons. Stir frequently, taking care not to burn the sugar. Bring to a boil, add the pectin-sugar mixture, and maintain a rolling boil. Skim away any foam that forms; if there is too much foam, add the butter.

Test for set:

Once a boil has been reached, take the temperature with a quick-read thermometer. Continue boiling and stirring until the mixture consistently reads 220F on a candy thermometer for one full minute. Turn off the heat. Remove the lemon slices.

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 jam. Turn the heat under the canner to high. Use a ladle to pour the jam into the jars through a canning funnel, leaving 1/2" 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" 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 five 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.