Prepare fruit & macerate: Wash, hull, and quarter strawberries. Wash and chop rhubarb into ½-inch pieces. Combine strawberries, rhubarb, sugar, and lemon juice in a large pot. Stir and let sit for 30 minutes to overnight.

Sterilize jars & lids: Wash jars, lids, and bands in hot, soapy water. Sterilize jars in boiling water for 10 minutes.

Cook the jam: Bring the fruit mixture to a rolling boil over medium-high heat, stirring frequently. Add optional butter and continue boiling for 15-20 minutes until set.

Fill jars: Remove from heat. Ladle hot jam into hot, sterilized jars, leaving ¼-inch headspace. Wipe jar rims clean and place warm lids before screwing on bands finger-tight.

Process jars: Place filled jars in boiling water, processing for 10 minutes. Turn off heat and let sit for 5 minutes before removing.

Cool & store: Place jars on a wire rack for 12-24 hours. Check seals and store sealed jars in a cool, dark place.