Preheat oven to 400 F degrees.: The moment you set the oven, you should notice the quiet hum of warming metal and the faint scent of dry air as the oven comes up to temperature. A properly preheated oven ensures the air inside is hot and consistent, which helps the panko crust brown evenly rather than slowly absorb oil and become soggy. One common mistake is putting the chicken in too early, which can cause uneven browning, so wait until the oven reads steady 400 F on your thermometer or display. You'll want your rack in the middle position for best circulation.
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.: Right away you'll notice the sharp, bright scent of the rice vinegar softened by the sugar and deepened by toasted sesame oil. The grated ginger and garlic add a lively aromatics layer, while the diced cucumber will begin to exude a faint wetness as it marinates. This resting time lets the salt and sugar draw out moisture and fuse the flavors, so the relish tastes cohesive rather than disjointed. A frequent pitfall is over macerating the cucumber, which can make the relish watery, so stir gently and keep the pieces evenly sized. Texturally, the relish should remain crisp and slightly glossy.
Season chicken cutlets with 1/4 teaspoon salt and pepper. Place flour on a shallow plate and egg in a medium bowl. Place panko, Togarashi, 1/4 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.: As you season and set up your dredging station, you'll sense the ritual rhythm of assembly. The light dusting of flour creates a dry surface for the beaten egg to grab onto, and the panko mixed with hichimi Togarashi and sesame seeds forms the crunchy exterior that will crisp in the oven. When you press the panko onto the cutlet, you should feel it adhere without packing it down, which preserves air pockets that brown. On the sheet pan, the sprayed oil will begin to heat and sizzle around the edges, producing that toasty aroma mid bake. Rotate the pan at the halfway mark so the browning is uniform; if one side gets too dark it indicates uneven heat, which you can fix by repositioning the pan. Avoid crowding the pan, or the cutlets will steam and fail to crisp properly.
Top each cutlet with about 1/2 cup relish, 1 teaspoon soy sauce and sriracha (if desired). Serve.: The first sensory cue here is contrast, the warm, crunchy crust meeting the cool, fragrant relish. Spoon the relish so it sits atop the baked crust rather than soaking into it, preserving the texture contrast. The reduced sodium soy sauce adds a glossy savory finish and umami echo; if you use sriracha sparingly you'll add a vinegary heat that lifts the flavors without overwhelming them. A common misstep is adding the relish too early, which softens the crust, so wait until you're ready to serve. On the plate, the dish should present a lovely interplay of color, with pale green cucumber against golden panko, and you should hear a gentle crunch when you cut into the cutlet.