Boiling Water Bath Jam for 5 Pints

Instructions:

1. Place clean jars in the water bath canner, fill with water, and bring to a simmer to keep the glass hot.
2. Combine smashed strawberries, diced rhubarb, and lemon juice in a heavy-bottomed pot.
3. Whisk the powdered fruit pectin into the fruit mixture.
4. Turn heat to high and bring the mixture to a rolling boil.
5. Stir in the granulated sugar all at once and add the butter to reduce foaming.
6. Maintain a rolling boil for exactly one minute, stirring constantly.
7. Ladle the hot jam into warm jars, leaving 1/4 inch of headspace, and remove trapped air bubbles with a bubble remover tool.
8. Wipe rims clean, center the lids, and screw bands until finger-tip tight.
9. Lower jars into the canner and process in boiling water for 10 minutes, ensuring water covers tops by 1-2 inches.
10. Remove jars and let them sit undisturbed on a towel for 24 hours to seal.