Add the soy sauce, sesame oil, ground ginger, sugar, and cornstarch to a small bowl. Whisk thoroughly until the cornstarch is dissolved and the sauce is smooth.
Prepare the vegetables. Cut the carrots into thin matchsticks, separate the broccoli into small florets, slice the red bell pepper into strips, and rinse and dry the spinach.
Lightly grease a large skillet or wok and place it over medium-high heat. Add the chicken strips and cook for 3–4 minutes, stirring frequently, until the outside is no longer pink.
Add the carrots, broccoli, and red bell pepper. Stir fry for 5–8 minutes, keeping everything moving regularly, until the vegetables begin to soften but still have some bite.
Add the spinach and prepared sauce to the skillet. Reduce the heat to medium-low and cook for about 5 minutes, stirring frequently, until the spinach has wilted, the sauce has thickened, and the chicken is fully cooked.
Remove the 20 Minute Chicken Stir Fry from the heat. Stir in 1–2 teaspoons of fresh lemon juice, if desired.
Spoon the chicken and vegetable stir fry over freshly cooked rice and garnish with sesame seeds. Serve warm.