The beauty of this jam is the quickness of both making it and—happily—eating it. You’ll see. I use blueberries, strawberries, and raspberries for this version, but the same method applies to a single-fruit jam or a combination of others, such as blackberries or gooseberries. I like adding strawberries, as they hold their shape slightly better and give the jam thickness. Blueberries are good for their color, and raspberries for their texture. The general principle is 1 pound of fruit for 1 cup of sugar. I then add lemon, orange, and lime zest; a few tablespoons of each citrus juice; and a pinch of salt to bring out the sweetness of the fruit. Then it simmers gently until slightly soft. That’s that—simple, easy, and full of flavor: summer in a jar. This jam is looser and runnier than the kind you’d find at the store. It’s just as delicious served as …