Mix together marinade, and mix together cornstarch paste.
Cut the fish into 1/2-inch slices and put in a bowl with marinade; mix well and set aside.
Drain and squeeze the mushrooms; remove the stems and slice the caps.
Cut the red and green peppers into 3/4-inch strips, and then diagonally into lozenge shapes.
Cut the scallion greens into bite-sized pieces.
Heat the wok over high heat, then add some peanut oil. When hot, add the fish and stir with chopsticks to separate the slices; when they are white but not fully cooked, remove with a slotted spoon and set aside.
Drain off all but three tablespoons of the oil, and return the wok to the stove top over high heat; add the garlic, ginger, mushrooms and peppers and stir-fry until fragrant; add the fish and season with soy sauce, vinegar and salt to taste; add the stock, and then the cornstarch paste, stirring as it thickens the liquid to a glaze.
Stir in the scallion greens and then, off the heat, the sesame oil. Serve.