Prepare fruit: hull strawberries and chop rhubarb.
In a large pot, combine fruit, lemon juice, pectin, and butter.
Bring to a boil, stirring constantly.
Add sugar gradually; stir until dissolved.
Bring mixture to a rolling boil for 1 minute.
Skim foam.
Ladle hot jam into sterilized jars, leaving 1/4-inch headspace.
Wipe rims, seal with lids, and process in a water bath for 5 minutes.
Cool and store.
Notes
– Use fresh fruit for best flavor.
– Add butter to reduce foaming.
– Adjust sugar to taste, but keep enough for proper preservation.