Combine the soy sauce, mirin, sake, sugar, onion, ginger, and garlic in a saucepan.
Bring to boil over high heat, stirring constantly.
Reduce heat to a simmer and cook for about ten minutes.
Strain out and discard solids.
Put liquid into a sealable, leakproof, container.
Thinly slice radishes on a mandolin. Store in a resealable plastic bag.
Finely chop green onions. Store in a resealable bag or container.
At the Campsite
Trim fat off of chicken and season both sides with salt and pepper.
Heat oil in a skillet over high heat until it starts to shimmer. Cook chicken undisturbed on one side for about 5 minutes.
Lower heat to medium, flip chicken and cook for an additional 5 minutes.
In a small bowl, mix two teaspoons of cornstarch with two teaspoons of water.
Pour teriyaki sauce (that you made at home) into the pan with your chicken, along with cornstarch slurry. Stir with spatula and continue cooking, continually flipping the chicken in the teriyaki/cornstarch mixture until well coated and mixture has thickened (1 to 2 minutes).
Chop romaine into 1 inch ribbons. Top with sliced radishes, then cooked chicken. Add additional teriyaki sauce, then sesame seeds and green onions.