Working over a medium bowl, use a vegetable peeler to create long, thin strips of cucumber. Add the radishes, scallions, cilantro, vinegar, sesame oil, 1/4 teaspoon of the salt, and the black pepper and toss. Cover and refrigerate.
Using the flat side of a knife or the bottom of a heavy pan, crush the coriander seeds and peppercorns.
Heat the canola oil in a large skillet over medium-high heat. Sprinkle the remaining salt over the tuna and press some of the crushed coriander and pepper onto each side. Cook, turning once, until the tuna reaches the desired doneness, about 3 minutes per side for medium-rare. Divide the tuna among individual plates and serve with the Asian slaw.
Tip: Coriander adds a fresh, almost sweet flavor to many dishes. Crush some extra coriander seeds and black peppercorns and use the mixture as a rub for steaks, pork chops, or firm fish, such as halibut and salmon.