Wash and chop rhubarb and strawberries into small pieces.

Add fruit, sugar, and lemon juice to a large heavy-bottomed pot.

Cook over medium heat, stirring frequently until sugar dissolves and fruit releases liquid.

Bring mixture to a steady boil and continue cooking for 30-40 minutes, stirring often.

Skim off excess foam if needed during cooking.

Test jam by placing a spoonful on a cold plate. If it wrinkles slightly when pushed, it is ready.

Ladle hot jam into sterilized jars, leaving 1/4-inch headspace.

Wipe jar rims clean, apply lids, and process in a boiling water bath for 10 minutes if canning for shelf storage.

Let jars cool completely before storing.