To prepare salsa, combine all ingredients in a bowl and toss lightly to combine flavors. Rest at room temperature for 30 minutes. Rub fish with oil and season with salt and pepper. Place on a medium-hot grill and cook on both sides until just cooked. Spoon salsa over fish.
If your plums are a little tart, add a pinch of brown sugar. If hot salsa is your thing, add some minced habanero pepper