Preheat oven to 400°F.
In a dutch oven, melt butter over medium-high heat. Add olive oil and stir. Add onion and garlic and cook for 5 minutes or until onion is translucent. Add celery and carrots and cook for 3 minutes or until softened. Add parsley and thyme and cook for 1 minute.
Add wine and cook until liquid is reduced by about half. Add flour and mix until mixture thickens. Add broth, salt, pepper and mustard seeds. Cook for 3 minutes. Add heavy cream, stir to combine and cook for 2 minutes. Turn off heat.
In a small bowl, whisk egg and add water.
Scoop filling into desired pie pan(s) and top with pastry dough. Crimp pastry dough along the sides to make it stick to the pie dish.
Brush dough with egg wash, making sure to brush along the sides. Using a knife, cut four small slits in the middle of the dough, as this will allow for steam to release from the pie.
Place on a baking sheet and cook in oven for 20 minutes or until pastry dough is golden brown.
Remove from oven, top with flaky salt (optional) and cool for 5 minutes before diving in.