Preheat the oven to 425˚F.
In a large saucepan, melt the butter. Add in the onions and cook until transparent, then add in the oregano and flour.
Add in the gravy, water and milk. Stir for around 5 minutes, then add in the peas, corn and turkey. Heat until the vegetables are tender, around 5 minutes.
Pour the mixture into the pie crust, and place the other pie crust on top. Press them together. Cut slits on the top of the pie.
Place on a baking sheet, and bake in the preheated oven for around 45 minutes. Remove from the oven, and allow to rest for around 10 minutes before serving.