Preserving time and energy in the pursuit of fresh flavor is the triumph of these jams and marmalades. For speed, start with commercial preserves; they hold up when heated and reheated. Then add fresh fruit to personalize the results. Boiling the two together gives you homemade taste in minutes. And the product is hot enough to be sterile without any processing. Return the mixtures to the preserves' original jars. If you misjudge the consistency during cooking, a simple step saves the day: Tilt the jars of cool jam or marmalade. If the mixture is runny, return it to the pan, bring it to a rolling boil, stir often, and test again for firmness about every minute.