Working over a medium bowl, use a vegetable peeler to create long, thin strips of cucumber.
Add the radishes, scallions, cilantro, vinegar, sesame oil, and ¼ teaspoon each salt and black pepper and toss. Cover and refrigerate.
Heat the canola oil in a skillet over medium-high heat. Using the flat of a knife or the bottom of a pan, crush the coriander seeds and peppercorns.
Sprinkle 1 ¼ teaspoons salt over the tuna and press the crushed spices onto each side.
Cook the tuna, turning once, until it reaches desired doneness, about 3 minutes per side for medium-rare.
Divide the tuna among individual plates and serve with the slaw.