Prepare the Sauce: Heat a large saucepan over medium heat and lightly coat with cooking spray or a small amount of oil. Cook the diced onion in a skillet for 2-3 minutes until it becomes soft and translucent. Stir in the garlic and ginger, cooking for an additional minute until fragrant. Add the zucchini, butternut squash, and curry powder, stirring well to coat the vegetables in the spices. Stir in the tomato paste, soy sauce, Chinese five-spice powder, and chicken stock. Bring the mixture to a boil, then reduce the heat to simmer for 20 minutes or until the vegetables are soft. Blend the sauce with an immersion blender or in a food processor until smooth. Set aside.

Cook the Curry: Heat a separate pan over medium heat, spraying it lightly with oil. Add the diced chicken breast and cook until browned on all sides, about 5-7 minutes. Remove the chicken and set aside. In the same pan, sauté the chopped onion and green bell pepper for 3-4 minutes until softened. Return the chicken to the pan and pour the prepared curry sauce over the mixture. Simmer on low heat for 10 minutes.

Serve: Serve the curry hot over steamed rice, noodles, or alongside a side of naan bread. Garnish with fresh cilantro if desired.