Dry the scallops well with paper towels and season with 1/2 tsp. salt and the cayenne. In a 12-inch cast-iron skillet, heat 1 Tbs. of the oil over high heat until hot. Cook the scallops, turning once with tongs, until golden brown and just opaque throughout, 3 to 4 minutes per side. As they finish cooking, transfer the scallops to a large plate and brush the top of each with hoisin sauce.
Wipe out the skillet and then heat the remaining 1 Tbs. oil over medium-high heat. Add about two-thirds of the scallions and cook, stirring constantly, until softened, about 30 seconds. Add the spinach, cilantro, and 1/4 tsp. salt and cook, tossing constantly with tongs, until just barely wilted, about 2 minutes. Remove the skillet from the heat, drizzle the spinach mixture with the sesame oil, and toss to combine well.
Serve the scallops with the spinach, sprinkled with the remaining scallions.