Place the chicken thighs in a medium bowl. Add 2 tablespoons of the sake and the cornstarch; stir well with chopsticks or tongs to evenly coat the chicken. Cover and refrigerate while you cook your rice.
Rinse the rice in a fine-mesh sieve until the water runs clear. Place the rice in a medium saucepan and add 1 1/2 cups of water (or follow the ratio and cooking instructions on your rice package). Bring to a simmer over medium-high heat. Reduce the heat to low, cover and cook for 20 minutes.
Meanwhile, halve the onion and cut into thin slices. Add the instant dashi to 1/2 cup of hot water and stir until it dissolves.
Once your rice is cooked, remove from the heat and keep warm.
Preheat a large skillet over medium-high heat. Add the peanut oil and swirl to coat. Add the chicken in one layer and cook until browned on both sides, about 4 minutes total. Remove the chicken from the skillet and reserve on a plate.
Next, add the onions to the skillet, reduce the heat to medium and cook, stirring, until slightly soft, about 1 minute.
Add the mirin, soy sauce, sugar, dashi mixture and remaining 1/4 cup sake to the skillet. Stir and scrape the bottom of the skillet to release any browned bits.
Increase the heat to medium-high. Bring to a simmer and add the chicken along with any juices from the plate to the skillet; cook, stirring occasionally, until the liquid reduces to a glaze, about 3 minutes.
Meanwhile, thinly slice the scallions, keeping the light and dark green parts separate. Add the light green scallions to the skillet and stir.
Place the rice in individual serving bowls. Divide the chicken yakitori over the rice, garnish with the dark green scallions and serve.