How to Make It
In a small skillet, simmer the orange juice over moderate heat until reduced to 2 tablespoons, 6 to 7 minutes. Transfer to a blender. Add the cooked egg yolk, rice vinegar and mustard and process until smooth. With the machine on, add 1/3 cup of the olive oil in a thin stream and process until emulsified. Season with salt and pepper and transfer to a small heatproof bowl. Set the bowl over a small saucepan of barely simmering water and keep the sauce warm; do not boil.
Wipe out the skillet. Add the canola oil and heat until shimmering. Add the capers and cook over moderately high heat until they open slightly and begin to brown, about 4 minutes. Using a slotted spoon, transfer the capers to paper towels to drain.
Using a sharp knife, peel the grapefruit, removing all the bitter white pith. Working over a bowl, cut in between the membranes to release the segments into the bowl.
In a small bowl, combine the ground coriander and cumin. Season the scallops with salt and pepper and dip half of each scallop into the spice mixture to coat it. In a large skillet, heat the remaining 2 tablespoons of olive oil until almost smoking. Add the scallops in a single layer and cook over high heat until golden, about 3 minutes. Turn the scallops and cook for 1 minute longer. Remove from the heat.
Spoon 2 tablespoons of the warm sauce onto each of 4 plates. Arrange the scallops on the sauce and place the grapefruit segments around them. Scatter the capers and herbs over the scallops and serve right away.