In a small bowl, whisk together vinegar, sugar, sesame oil, ginger, garlic, and salt. Add cucumber, onion and sesame seeds and stir. Set aside to allow flavors to combine.
Season chicken cutlets with ¼ teaspoon salt and pepper. Place flour on a shallow plate and egg in a medium bowl. Place panko, Togarashi, ¼ teaspoon salt, pepper and sesame seeds on another shallow plate. Dredge each cutlet lightly in flour, then egg (shaking off excess), then panko mixture. Place on a sheet pan covered in parchment. Spray cutlets lightly with olive oil and bake for 20 minutes, rotating pan halfway through.
Top each cutlet with about 1/2 cup relish, 1 teaspoon soy sauce and sriracha (if desired). Serve.