Neat compartments of teriyaki chicken, rice, and pickled veggies make this bento box a high protein lunch that's as organized as it is delicious.
Mix soy sauce, mirin, sake, and brown sugar in a small saucepan. Simmer over medium heat until slightly thickened, about 5 minutes, stirring frequently. Set aside—the sauce should coat the back of a spoon.
Pat chicken thighs dry and slice into 2-inch strips. Season lightly with salt. Heat vegetable oil over medium-high heat in a large nonstick skillet. Sear chicken 3–4 minutes per side until golden and just cooked through.
Pour half the teriyaki sauce over the chicken in the skillet. Reduce heat to medium-low and toss chicken to coat as the sauce bubbles for 1 minute. Remove from heat once glazed and sticky (avoid overcooking or the sauce will scorch).
Toss sliced cucumber and julienned carrot with rice vinegar and a pinch of salt in a small bowl. Let stand 10 minutes, stirring once or twice, to lightly pickle.
Assemble each bento: Spoon 3/4 cup rice in one compartment, arrange teriyaki chicken beside. Add pickled cucumber and carrot, and sprinkle with chopped scallions and sesame seeds. Drizzle with extra teriyaki sauce if desired.