In a large pan, heat the vegan butter over medium heat, then saute the chopped leeks for about 5 minutes until they start to soften and brown. Add the thyme, oregano, salt and pepper, and stirfry for 1-2 more minutes to wake up the spices.
Next, add the potatoes, and stir/cook for a few minutes to slightly soften and brown them. Then add the plant-based chicken and stir well.
Finally, add the water, nutritional yeast, soy sauce, and cornstarch, and mix it all in well. Remove from heat and stir in the peas. Taste and adjust for salt and pepper. Set aside to cool the mixture while you roll out the crust.