Place diced plums and sugar in a medium saucepan (I used cast-iron here) over medium heat. Bring the mixture to a simmer, stirring to dissolve the sugar. Reduce heat to medium-low, and continue to cook, stirring occasionally, until the mixture thickens and some of the plums break down (20 - 25 minutes).

Take the mixture off the heat, and taste for sweet-tart balance. If the mixture is too sweet, add a squeeze of fresh lemon juice. Taste, and repeat until the balance is to your liking. Let cool to room temperature, transfer to a clean container, and store in the refrigerator. Make sure the compote is well-chilled before using with the panna cotta. Enjoy any leftovers with pancakes, granola, ice cream or Greek yogurt.