1. Pit peaches and plums and coarsely chop into pieces slightly smaller than 1/2 inch. Combine fruit, sugar, water, 1 tablespoon lemon juice, and salt in a heavy-bottomed pot and bring to a boil over medium-high heat. Boil (do not stir) until mixture begins to thicken, about 18 minutes.

2. Reduce heat to low and cook, stirring frequently, until mixture is thickened and jamlike, about 30 minutes. Add more lemon juice to taste, then let cool completely.