In a large, non-reactive pot or Dutch oven, combine the strawberries, rhubarb, and sugar. Stir gently to combine.

Let the mixture sit at room temperature for at least 30 minutes (or up to 2 hours) to allow the fruit to release its juices and dissolve some of the sugar.

Place the pot over high heat and bring the mixture to a rolling boil, stirring frequently to prevent scorching. Cook for 5 minutes.

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

Remove the pot from the heat. Skim off any foam from the surface using a spoon. Test the set point if desired (a cold plate test). If it reaches setting point, proceed to jarring.

Carefully ladle the hot jam into sterilized pint canning jars, leaving 1/4 inch headspace. Wipe the rims clean, affix lids and bands, and process in a boiling water bath for 10 minutes (adjusting for altitude) for shelf stability.