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5 cups rhubarb (about 1 1⁄4 lbs.), cut into 3⁄4″ x 1⁄2″ cubes
2 cups hulled and quartered strawberries (about 1⁄2 lb.)
2 1⁄4 cups sugar
1 tbsp. fresh lemon juice
1. Combine the ingredients in a 4-qt. saucepan over medium heat. Bring to a boil and reduce heat to medium-low; cook, stirring occasionally, until the rhubarb breaks down and the jam has thickened, about 1 hour. To determine whether jam has set, place a small spoonful on a chilled plate; if the dollop of jam holds firm and doesn’t get runny around the edges, it is ready for canning. If it runs, continue to cook for another 10 minutes.
2. Meanwhile, submerge three 1-cup canning jars, along with their lids and ring bands, in a large pot of boiling water and sterilize over high heat for 10 minutes. Transfer sterilized jars, lids, and bands to a clean dish towel. Fill each jar with hot jam, leaving at least 1⁄4″ of space at the top. Wipe jar rims with a clean dish towel, place lids on jars, and secure ring bands.
3. Transfer filled jars to a canning rack; place rack in a pot of gently boiling water so that jars are submerged by at least 1″; let boil for 10 minutes. Transfer jars, set at least 1″ apart, to a dish towel and let cool, undisturbed, for 24 hours. To test that jars have properly sealed, unscrew bands and lift each jar by the edge of the lid; if the lid holds, the jar is sealed. If it loosens, jar is not fully sealed, and jam should be refrigerated and used within 2 weeks. Sealed jars will keep, in a cool, dark place, for up to a year.