Set a large, heavy skillet over medium-high heat for 1 minute. Add 1-1/2 Tbs. of the oil, swirl it in the skillet, and add the fennel. Season with 1/2 tsp. salt and a few grinds of pepper. Cook, stirring occasionally, until the fennel wilts and starts to brown, about 4 minutes. Add 1/4 cup water to the skillet, cover, reduce the heat to medium, and continue to cook, stirring occasionally, until the fennel is just tender, about 4 minutes. Stir in the spinach and toss until it wilts, about 1 minute. Remove from the heat and keep warm.
While the fennel is cooking, slice the ends off the orange with a small, sharp knife. Stand the orange on one of its cut ends and slice off the skin in strips (try to get all the bitter white pith). Working over a small bowl, cut the orange segments free from the membrane, letting each segment fall into the bowl as you go. Squeeze the empty wheel of membranes over the bowl to release any remaining juice.
Set a large cast-iron pan or a heavy 12-inch skillet over high heat for 1-1/2 minutes. Season the scallops with 1/2 tsp. salt and a few generous grinds of pepper. Add the remaining 1-1/2 Tbs. oil to the skillet, swirl it in the pan, and add the scallops. Cook without moving for 3 minutes. Gently flip the scallops and cook until they’re well browned and almost completely firm to the touch, 1 to 2 minutes. Sprinkle the scallops with the thyme. Portion the fennel mixture onto four dinner plates, top with the scallops and orange segments, and drizzle with the orange juice. Serve immediately.