To prepare the salmon, place the salmon skin-side down on a cutting board, then make parallel cuts lengthwise, slicing down toward the skin without cutting through, about ½ inch apart, then turn and slice crosswise at an angle to create a crosshatch (blooming) pattern, then sprinkle salt and white pepper over the salmon and coat evenly with cornstarch, making sure it gets into the cuts.
To fry the salmon, in a pot over high heat heat the cooking oil to 325°F (163°C), then carefully add the salmon and fry for about 3 minutes until golden and crispy, then remove and drain.
To make the teriyaki sauce, in a saucepan over medium heat, combine the chicken stock, soy sauce, sake, and sugar, and bring to a simmer. Then, in a small bowl, mix the cornstarch with water to form a slurry, stir it into the sauce, and cook until thickened. Lastly, add the scallions and stir to combine.
To serve, place the crispy salmon on a serving plate, then drizzle the teriyaki sauce over the top or serve it on the side, garnish with scallions, and serve immediately.