How to Make It
Preheat oven to 275°F. Very thinly slice half of the oranges. Pile the slices on a large rimmed baking sheet, and drizzle with 1 tablespoon oil. Season with a big pinch of fine salt and several grinds of pepper, and toss. Spread the orange slices in a single layer, overlapping as needed.
Season the salmon fillet generously with 1 teaspoon fine salt and 1/4 teaspoon pepper, and place it on top the oranges in the center of the pan, skin side down. Drizzle the salmon with 1 tablespoon pomegranate molasses; use the back of a spoon to spread it over the top of the fillet, coating it evenly. Sprinkle with black sesame seeds.
Bake in preheated oven until salmon is medium-rare, which can take as little as 15 minutes for a thinner fillet or up to 30 minutes if it’s quite thick. To determine doneness, insert a metal cake tester or the tines of a fork into the center of the thickest part of the fillet for about 5 seconds, then remove it and place it against your lip. If it is pleasantly warm, the salmon is done. (If it is hot, the salmon is overcooked.)
Meanwhile, peel and segment any remaining mandarins, and supreme the larger remaining oranges. To do this, cut a slice from the top and bottom of an orange to reveal the flesh. Stand the orange upright and slice away the peel in wide strips, cutting downward, following the contour of the fruit, and removing all the white pith and as little of the flesh as possible. Holding the orange over a medium bowl, use a sharp paring knife to cut along both sides of each segment, releasing the segments and allowing them and the juice to drop into the bowl beneath. Pick out the seeds as you go. Squeeze any remaining juice from the pith into the bowl. Repeat with the remaining oranges.
To the bowl of orange supremes, add the mandarin segments, herbs, olives, and remaining 1 tablespoon oil. Season with 1/2 teaspoon fine salt and a pinch of pepper, and set aside.
When the salmon is done, immediately drizzle it with remaining 1 tablespoon pomegranate molasses. Arrange a few of the roasted citrus slices on a platter as desired; transfer the salmon fillet to platter. Spoon the salad over and around the salmon. Drizzle everything with olive oil, and sprinkle with flaky salt.