1. Make the dressing: In a small bowl, whisk mirin, rice vinegar, soy sauce, and horseradish. Set aside.
2. Heat a large skillet over medium-high heat, then add oil. When the oil is hot, add ginger and garlic and sauté until softened, about one minute. Add tuna, and cook, stirring frequently, until fish is opaque and cooked through, about three to four minutes. Add dressing and remove from heat.
3. In a large bowl, mix lettuce, scallions, radishes, sugar snap peas, and toasted sesame seeds. Pour tuna with dressing over salad right before serving. Toss and serve immediately.
Tips: Make ahead: Tuna and dressing can be made ahead of time. Store, covered, in the refrigerator. Prepare salad and toss with dressing right before serving.