• Cut turkey breast into 1-cm (½-in.) cubes and put in refrigerator.
• Heat oil over medium-high heat in a large non-stick skillet (if the skillet isn't big enough, use two skillets or perform step in two batches). Add mushrooms, celeriac, garlic, salt, and dried thyme. Cover and cook for 10 minutes, stirring from time to time. Set aside in a bowl.
• Wipe out skillet, melt 45 mL (3 tbsp.) butter over medium heat, add leeks, cover, and cook for 5 minutes. Add a pinch of salt.
• Add white wine and reduce until evaporated.
• Add flour, stir well, and cook for 2 minutes.
• Add cream and poultry broth and simmer, stirring, for 8 minutes.
• Add reserved mushroom mixture and mustard.
• Mix well, taste, and adjust seasoning as needed. Pour into a bowl and set aside.
• In the same skillet, heat 15 mL (1 tbsp.) of butter over high heat and fry turkey cubes until fully cooked. Season with 2 mL (½ tsp.) each of salt and dried thyme, and season with pepper to taste. Cook turkey cubes in two batches if needed so they brown well.
• Add turkey cubes to the mushroom mixture and reserved leeks.
• Transfer mixture to a 22.5 cm x 30-cm (9 x 12-in.) baking dish or into 8 individual 500-mL (2-cup) baking dishes (see notes).
• Sprinkle with cheddar cheese, if using.
• Brush the edge of baking dish or dishes with the egg yolk mixture.
• Top with pastry dough to cover entire surface, leaving an overhang of 1 cm (1/2 in.). Press gently with your fingers to seal edges. Brush top of pie with egg yolk mixture.
• If you like, use small cookie cutters to shape leftover dough into decorative pieces. Place on top of pie and brush with egg yolk mixture.
• Bake in centre of a 200°C (400°F) oven for 20 to 30 minutes depending on the size of baking dishes.

Ready to entertain

Get a head start by freezing the filling in freezer bags. Thaw it in the fridge 24 hours ahead of time. Then just transfer to a baking dish, cover with dough, and bake the pie.

NOTES

• You can also find frozen pastry at the grocery store that you can roll out according to your needs.
• For individual-size servings, cut the pastry dough into circles or squares about 1 cm (½ in.) larger than the diameter of the baking containers.

Tips:

Choose firm celeriac, slice off the two ends with a chef's knife, peel, cut into ¼-inch slices, and then into cubes.