Bring water to boil in medium sauce pan.
Heat heavy skillet to medium heat.
Boil potatoes in water for about 5 minutes or until fork tender.
Add oil, chicken, thyme, marjoram, salt, and black pepper to preheated skillet. Cook 7-8 minutes or until mostly done with no more pink showing, stirring halfway through (will continue to cook in the oven).
Remove chicken from the pan.
Melt butter in the skillet (still on medium heat). Add mushrooms and leeks. Stir, cover, and cook 5 minutes.
Uncover and reduce heat to low-medium (about 2-3). Add flour (sprinkle over top) and stir in. Add stock and stir. Add cream and mustard and bring to a simmer (not rolling boil). Increase the heat a bit if needed, but watch constantly so mixture doesn't go beyond a bubbling simmer.
When thickened, add parmesan cheese, and turn off the heat. Add the potatoes and chicken and stir.
Pour filling into individual ramekins or large pie pan. Cover with pastry to fit top of pan(s). Cut small slit in each small pie or several in larger pie. Brush with melted butter.
Place on sheet pan to catch any overflow.
Bake for 30 minutes or until pastry is nicely browned and filling is bubbly.