Remove and discard fibrous light-green stems from the fennel bulb. Reserve 2 tablespoons dark-green feathery leaves. Quarter the white bulb and slice thinly.
Heat oil in a large skillet over medium-high heat. Add the onions and fennel, and cook until the onions are golden, 4 to 6 minutes. Add the garlic and cook for 1 minute. Grate the zest from half of the orange into the pan. Cut the orange in half and squeeze all of its juice into pan. Stir in the balsamic vinegar and simmer until the liquid reduces slightly, about 2 minutes. Stir in honey, raisins, red pepper flakes, mint, and tomatoes. Reduce heat to medium-low and cook until the flavors blend, 5 to 8 minutes.
Preheat oven to 400ºF. Spread a few tablespoons of the sauce over the bottom of a 2 1/2-quart baking dish. Lightly season the fish with salt and pepper. Arrange the fish in the prepared dish and scatter on the olives. Spoon the remaining sauce over the top. Bake uncovered until the fish is just a bit filmy and moist in the center when checked with a fork, about 15 minutes.
Serve the fish with juices from the pan and the reserved fennel leaves.
Tip: Serve this easy fish dinner with pieces of Panera Bread Asiago Cheese Focaccia for dipping into the juices from the pan.