To make the dressing, mix the canola oil, sesame oil, rice vinegar, mirin, soy sauce, and lime juice. Whisk in the miso until smooth.
To make the salad, toast 1 tablespoon of the white sesame seeds in a dry frying pan over medium heat, stirring constantly, until golden brown, 2 to 3 minutes. Transfer to a plate to cool.
Mix the remaining 2 tablespoons white sesame seeds with the black sesame seeds and coriander seeds on a plate. Brush the pieces of salmon lightly with oil all over, then dredge them in the sesame and coriander mixture. Make sure all sides are properly coated.
Heat a dry frying pan over high heat. Sear the salmon for about 30 seconds on each side until the coating is slightly caramelized.
Remove the fish from the pan and immediately cut into ½-inch (13 mm) slices, to stop the cooking process.
Toss the cucumber, carrot, scallion, radish, and soybeans with half of the dressing and toasted white sesame seeds in a medium bowl. Top the salad with the salmon and garnish with radish greens (if using).
Serve the remaining dressing on the side.