Arrange one rack in the lower third and one rack in the center of the oven; preheat the oven to 400 F. Spray 1 large baking sheet and one small baking sheet with olive oil spray, or brush the sheets with olive oil.

Stir the fennel seeds in a small dry skillet over medium-high heat until fragrant, about 2 minutes. Transfer to a mortar or spice mill and grind coarsely.

Cut the potatoes in half lengthwise, then cut each pieces lengthwise into 4 wedges that measure about ¾ inch at the widest part. Place in a large bowl. Halve the fennel bulbs lengthwise, and then cut into wedges that measure about ½ inch at the widest part. Place in the bowl with the potatoes. Add 3 tablespoons olive oil, 2 tablespoons toasted fennel seeds, salt and pepper and toss to coat. Spread on the large baking sheet. Place on the center rack in the oven and roast until beginning to brown on the bottom and almost tender, about 35 minutes.

Meanwhile, prepare the fish and gremolata: Arrange the fish fillets skin side down on the small baking sheet. Sprinkle the fish with the remaining 2 teaspoons toasted fennel seeds, coarse kosher salt and freshly ground pepper. Dip a pastry brush in olive oil, and dab over the fish. Halve the kumquats lengthwise, then thinly slice lengthwise (it is easier to slice the cut side of the fruit than the skin side), discarding the seeds. Cut the slices finely crosswise, forming chopped kumquats. Sprinkle ½ of the chopped kumquats over the fish. Place the remaining kumquats in a small bowl; mix in the parsley and shallot to make the gremolata.

After the vegetables have roasted for 35 minutes, place the fish on the lower oven rack. Roast the fish and vegetables until the fish is just springy to the touch and opaque in the center and the vegetables are brown on the bottom and tender, about 10 minutes.

Transfer the fish and vegetables to plates; sprinkle with the gremolata and serve.