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.
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.
Once a boil has been reached, take the temperature with a quick-read thermometer. Continue boiling and stirring until the mixture consistently reads 220 degrees F on a candy thermometer for one full minute. Turn off the heat. Remove the lemon slices. If canning, process immediately using standard water bath method. Otherwise, allow to cool, then pack in jars and refrigerate, or freezer-safe containers or bags if freezing.
Just before processing, try giving yours a twist by adding one (or more!) of the following:
1 Tbsp real balsamic
A few fresh cracks of black pepper
2 Tbsp Campari A handful of mint or basil, finely chopped
Seeds from a vanilla bean
The zest of a lime or lemon