Combine strawberries, rhubarb, pectin, and lemon juice in a large saucepan or Dutch oven. Bring to a boil over high heat. Add sugar, stirring until dissolved.
Return to a rolling boil. Boil hard for 1 minute, stirring constantly. Remove from heat. Skim foam if necessary.
Ladle hot jam into hot, sterilized jars leaving ¼-inch headspace. Adjust two-piece caps. Process for 10 minutes in a boiling water canner. Makes about 6 half pints.
Recipe by The Culinary Cellar at https://theculinarycellar.com/ball-blue-book-of-preserving/