Position a rack about 7 inches from the broiler and heat the oven to 450°f.
Line the bottom of a broiler pan with foil and lightly oil the top of the pan.
Season the chicken all over with 1 tsp. salt and 1 2sp. pepper.
Arrange the shiitake in 8 pairs, gill sides up, on the prepared broiler pan and season with salt and pepper.
Arrange 2 or 3 scallion pieces on top of each mushroom pair, then put a chicken thigh, skin side up, on top. Press with your hand to flatten.
Roast until the edges of the chicken begin to brown and an instantread thermometer inserted in a thick part of the biggest thigh registers 165°f, about 20 minutes.
Turn the broiler to high and broil until the skin is crisp and deeply browned, 5 to 6 minutes, rotating the pan once for even browning.
While the chicken is cooking, combine the soy sauce, sugar, mirin, and orange zest in a small saucepan. Bring to a simmer over medium heat, stirring to dissolve the sugar.
In a small bowl, stir the orange juice and cornstarch; add this mixture to the saucepan.
Return to a simmer and cook, stirring constantly, until thickened and glossy, about 1 minute.
To serve, transfer the chicken, scallions, and mushrooms to dinner plates, drizzle with the sauce, and sprinkle with the sesame seeds.