Prepare water bath canner and jars according to standard sterile procedures. Keep jars hot until ready to fill.

Combine the cut rhubarb and strawberries in a large, heavy-bottomed non-reactive pot. Bring to a boil over medium-high heat, stirring constantly until the fruit begins to release juices (about 5-7 minutes). Mash lightly with a potato masher if a smoother jam is desired.

Stir in the entire package of powdered pectin and the lemon juice (if using). Return the mixture to a full, rolling boil that cannot be stirred down. Boil hard for exactly 1 minute, stirring constantly.

Remove the pot from the heat immediately. Carefully stir in the granulated sugar all at once. Continue stirring until the sugar is completely dissolved (this prevents scorching on the bottom).

Return the jam mixture to a full rolling boil over high heat. Boil hard, stirring frequently, for exactly 1 minute more. Skim off any foam that rises to the surface using a metal spoon.

Ladle the hot jam into the prepared, hot canning jars, leaving 1/4 inch headspace. Wipe rims clean, center lids, and screw on bands fingertip tight. Process in a boiling water bath for 10 minutes (adjusting time for altitude).

Remove jars and let cool completely on a towel undisturbed for 12-24 hours until sealed. Check seals, label, and store in a cool, dark place.