Combine all sauce ingredients in a small bowl. Mix well, add water to thin if needed.
In a large dry skillet/pan, toast sesame seeds for 30 seconds to 1 minute over high heat. Toss constantly while roasting to make sure they do not burn.
In same skillet or pan, heat cooking oil over medium heat. Stir in onions, bell pepper, garlic and chili pepper (if using). Cook for 3 minutes, stir regularly
Add tomatoes and cook for 3 minutes more. Stir regularly.
Then add zucchini noodles and cook until heated through. This will take about 3-5 minutes. Stir regularly.
Lastly, add sauce and stir well until coated and heated through.
Serve with toasted sesame seeds on top.