Mix lemon juice into bananas immediately after mashing to retain the fresh color then combine all the fruit and the pectin in a big pot. Bring to a boil, stirring frequently. Add sugar (and a dab of butter to reduce foaming). Bring to a rolling boil; boil hard one minute. Remove from heat and add nuts and rum (if using). Ladle into jars, leaving 1/4-inch headspace. BWB 15 minutes. Serve over ice cream with whipped cream and chocolate syrup!