Prep. Cook spaghetti as per package instructions but don’t overboil them as they will continue to cook later in the sauce.
Sauce. In a medium-size bowl, mix all the sauce ingredients until combined. Set aside.
Chicken marinade. In a medium-size bowl, add chicken breasts and pour 2 tablespoons of the sauce, cover with plastic wrap and let marinate for 1 hour. (If you’re in a rush, either make marinade a night before, and leave the chicken overnight to marinade or marinade as little as you have time, but definitely do eat as it will give extra flavor and tenderness to the chicken!)
Cook chicken. Heat up vegetable oil in a large skillet (or wok pan, but not necessary!) over high heat. Cook chicken for 3-4 minutes on both sides until slightly golden brown. Remove from heat and set aside.
In the same skillet, cook mushrooms (3-4 minutes) over medium heat, then add the rest vegetables and cook until bell peppers become softer but not too soft. (1-2 minutes).
Reduce the heat to low. Pour in the sauce, return chicken and noodles to the skillet and toss everything together until all of the noodles are brown and covered with sauce.
Garnish with green onions, sesame seeds, and serve while warm!