Dice 1 pound red plums; toss with 1 pound quartered strawberries, 3/4 cup sugar, 1 teaspoon vanilla extract, 1/2 teaspoon cinnamon and the juice of 1 lemon in a bowl. Cover and let stand 2 to 6 hours. Transfer to a large skillet. Add 1/3 cup honey; cook over medium-high heat, stirring occasionally, until the mixture is thick and registers 220 degrees F on a candy thermometer, about 30 minutes. Fill sterilized jars, leaving a 1/2-inch headspace, then seal and process for 15 minutes.
TIP: This jam is delicious with butter on a crusty baguette or drizzled over vanilla ice cream.