First, make sure your jam jars, lids and rings are clean and sterilized. You can sterilize them in the dishwasher, or by boiling them for a few minutes in a large stock pot.
In a large stock pot, combine the strawberries, rhubarb, lemon juice and zest. Simmer for about 15-20 minutes over medium heat, until the rhubarb is softened and beginning to break down. Mash the fruit slightly, or leave it chunky, whichever you like.
Sprinkle the pectin over the fruit and stir it in. Increase heat to medium/high and stir constantly (while wearing oven mitts to protect against splatter burns), and bring to a boil.
Dump all the sugar in at once. Use 1 1/2 cups of sugar for a more tart jam, or 2 cups for a sweeter jam. Continue stirring constantly to return to a vigorous boil that can’t be stirred down. Boil hard for 1 minute.
Remove from the heat and stir in the butter (the fat helps to reduce any foaminess).
Immediately ladle the jam into hot, sterilized jars, leaving a 1/4 inch gap at the top (called "head space") to allow for expansion. Wipe the rims of the jars with a clean damp cloth. Center the lids on the jars, and screw on the lids "fingertip tight".
Jam stored in the refrigerator should be consumed within 1-2 months.