Put the chopped rhubarb and chopped strawberries into a large, heavy-bottomed pot.

Add the sugar and 1/4 cup lemon juice. Stir to combine, then let the mixture rest for 30 minutes.

After resting, stir in the fruit pectin until it's fully dissolved.

Set the pot over medium heat and bring the mixture to a rolling, vigorous boil, stirring constantly.

Once at a rolling boil, reduce heat just enough to maintain that boil and continue for about 10 minutes, stirring frequently, until the jam thickens.

To test, chill a small plate in the freezer. Spoon a little jam on the cold plate; if it wrinkles when nudged, it’s ready.

Remove from heat and ladle the hot jam into sterilized jars, leaving 1/4 inch headspace.

Wipe rims clean, apply lids, and screw on bands fingertip-tight. Let jars cool completely on a rack.