Preheat the oven to 400F. Spread the cherries out in a single layer on a parchment-lined rimmed baking sheet. Whisk together equal amounts of honey and balsamic vinegar with about half as much oil (about 1/4 cup honey and balsamic and 2 Tbsp. oil for 1 L cherries) and pour over the cherries. Add a sprig of rosemary, if you like, and toss to coat. Grind over a bit of black pepper.
Roast for 10-20 minutes, stirring once or twice, until the cherries soften and release their juices and everything gets dark and sticky. Serve warm, over ice cream or a soft log of goat cheese, on a cheese board or good bread, or over thick plain yogurt and granola.