How to Make It
Pat scallops dry with paper towels. Set aside.
Melt 1/2 tablespoon of the butter in a large skillet over high until sizzling. Add half of the endive. Cook, stirring occasionally, until just beginning to soften, about 2 minutes. Transfer endive to a plate. Repeat procedure with 1/2 tablespoon of the butter and remaining endive. Return all endive to skillet. Sprinkle sugar, vinegar, and 1/8 teaspoon of the salt over endive, and continue to cook, stirring often, until endive is tender, about 3 minutes. Remove from heat and set aside.
Heat 1 tablespoon of the canola oil in a large cast-iron skillet over medium-high until shimmering. Add 8 of the scallops and press gently with a spatula. Cook, undisturbed, until browned, about 3 minutes. Turn scallops and cook 30 seconds or to desired degree of doneness. Transfer cooked scallops to a plate and wipe skillet clean. Repeat with remaining 1 tablespoon canola oil and remaining scallops. Sprinkle scallops with 1/4 teaspoon of the salt.
Cook remaining 2 tablespoons butter in a medium skillet over medium-high, stirring often, until butter turns golden brown, about 3 minutes. Remove from heat and stir in blood orange, hazelnuts, and remaining 1/2 teaspoon salt. Set aside.
Using a slotted spoon, drain endive and divide evenly among 4 plates. Top each plate with 4 scallops; spoon blood orange mixture evenly over scallops and endive. Top each with chopped navel oranges, and sprinkle with tarragon.