In a large nonstick skillet or wok, heat 1/2 teaspoon of the grapeseed oil over high heat. When the pan is hot and almost smoking, add the chicken, 1/2 tablespoon of the soy sauce, and the pepper flakes and brown the chicken, using a wooden spoon to break it into small pieces as it cooks, 3 to 4 minutes. Add the onion, garlic, and ginger and cook, stirring, until softened, 2 to 3 minutes. Transfer to a medium bowl.

To the pan, add the remaining 1/2 teaspoon grapeseed oil, the Brussels sprouts, and carrots. Cook, stirring, until browned, about 5 minutes. Add the bell pepper and mushrooms. Pour in the remaining soy sauce, rice wine, and sesame oil. Cook, stirring occasionally, until the vegetables are crisp-tender, about 2 minutes. Return the chicken to the skillet, stir, and reheat for about 30 seconds. Remove the pan from the heat, garnish with the scallion, and divide between 2 bowls.