Mix all the sauce ingredients in a bowl, except the cornstarch slurry. Feel free to adjust seasoning depending on desired taste. Set aside.
Heat a pot with water. Cook pasta until chewy or al dente.
While pasta is cooking, heat a pan. Add in 2 tbsp sesame oil. Once hot, sauté the onion and leeks (if using). Add in the garlic and bell peppers (if using) then sauté until tender. Add in the TVP/lentils/mushrooms then sauté for 2-3 minutes.
Pour in the sauce and leave to simmer over medium heat.
Once the sauce boils, add in the cornstarch slurry. Continuously mix until the sauce starts to thicken.
Add in the spaghetti and cook until it has absorbed some of the sauce. Season with a pinch of salt, or more as needed. Mix then turn off heat.
Top with some leeks and sesame seeds, if desired. Enjoy while hot!