Prepare the Filling:

Heat olive oil and butter in a sauté pan over medium-high heat.

Add leeks, celery, and a pinch of salt; reduce to medium heat and cook for 10-12 minutes until soft and translucent.

Pour in wine, increase heat, and cook until reduced by two-thirds.

Add mustard, onion powder, garlic powder, and chicken stock powder; cook 2 minutes more.

Remove from heat, fold in chicken and yogurt, then season with salt and pepper to taste.

Cool to room temperature, then chill in the fridge.

Make the Pastry:

Mix flour, suet, and salt. Add water and mix with a knife to form a shaggy dough.

Gently knead until dough forms, adjusting with extra flour or water if needed.

Grease a large pie dish with butter.

Assemble the Pie:

Divide dough: roll two-thirds for the base and one-third for the lid.

Line pie dish with base and cover both pieces with plastic wrap; chill for 3 hours.

Fill base with chilled filling, top with lid, and chill for 2-3 hours more.

Bake:

Preheat oven to 200°C.

Brush pastry with egg wash, chill for 5 minutes, and brush again.

Bake for 60-80 minutes until golden, crisp, and bubbling at edges.

Cool for 10 minutes before serving.