Slice the cucumber and green onions. Set aside.
Prepare rice according to the instructions on the package. Mix marin, sake, soy sauce, and sugar in a small microwave-safe bowl. Microwave for 30 seconds to melt the sugar. Stir and add fresh ginger.
Debone chicken thighs. Place thigh skin side down on a cutting board. Unroll and find the bone. Use your knife to score around the bone so it releases from the meat. Slip your knife under the bone and cut away from the chicken. Cut (skin side down) into 1.5 inch cubes. Season with salt. You can skip this and use boneless skinless thigh or chicken breast.
Heat oil in a high-rimmed pan over medium-high heat. Once the pan is hot, sear skin side down for 6-7 minutes, undisturbed. Once the chicken is nearly white around the top edges, flip and cook for 2-3 more minutes or until fully cooked through.
Discard excess juices, oil, and fat from the pan. Reduce heat to medium and add teriyaki sauce. Bring to a simmer and cook until thickened enough to coat the chicken, about 5 minutes. Flip chicken to coat all over.
Serve over rice with cucumber, green onion, and sesame seeds. Fine strain and serve with leftover sauce from the pan if desired.