Preheat oven to 450°F. Cut the stalks and fronds from the fennel and set aside. Cut the bulbs into quarters and remove the center cores (save in the freezer for making veggie broth later!). Slice into thin strips and set aside. Finely chop the fronds until you have about 2 tablespoons, and set aside.
Place the fish in a lightly oiled baking pan, in a single layer. Sprinkle with salt and pepper. Cover with the fennel stalks, then cover the pan tightly with aluminum foil. Set aside for just a few minutes while you get the fennel strips started on the stove.
In a large, nonstick pan, heat olive oil over medium heat. Add the onion and cook for a couple of minutes, until it begins to soften. Add the fennel and continue cooking for about ten minutes, until everything is tender.
At this point, put the fish in the oven. Bake for 15 minutes, then test for doneness. It should flake easily with a fork, and not be mushy in the center. If it is, bake another five minutes and check again.
Back on the stove, add the garlic. Stir and cover. Reduce heat, and cook for another 5 to 10 minutes, stirring occasionally. Add the chopped fronds, mix gently, and remove from heat. Season to taste with salt and pepper.
When the fish is done, remove from the oven and put the fennel stalks in your compost pile. Serve each piece of fish on a bed of the sautéed fennel.