In a medium saucepan, heat the mustard seeds and oil over low, stirring often, just until the seeds begin to pop, about 1 minute. Add the grapes, onion, cherries, sugar, vinegar, thyme, bay leaves, crushed red pepper and 2 tablespoons water, and bring to a simmer over medium-high heat. Adjust heat as needed to maintain a steady simmer and cook, stirring occasionally, until the fruit is tender and the liquid is syrupy, 40-45 minutes.
Stir in the mustard powder and Dijon. Remove from heat; let cool to room temperature. Discard the thyme sprigs and bay leaves. Serve the mostarda warm or at room temperature.
From Rachael Ray Every Day, November 2017