Preheat oven to 200°C/390°F. Lightly grease a 20cm (8-inch) pie dish.

Melt butter in a pan over medium heat. Add diced onion and cook 2 minutes until translucent.

Stir in crushed garlic clove for 30 seconds until fragrant.

Turn heat to medium-high, add sliced mushrooms, sauté 3 minutes until moisture evaporates and they begin to brown.

Sprinkle plain flour, stir constantly for 1 minute to cook out raw taste.

Gradually pour in chicken stock, whisking to avoid lumps. Stir in dried thyme. Simmer 1–2 minutes until sauce thickens.

Fold in cooked chicken and frozen peas. Season with salt and pepper. Spoon mixture into prepared pie dish.

Place potatoes in a saucepan, cover with cold water, season. Boil 15–18 minutes until tender. Drain and let steam 1–2 minutes to dry.

Mash potatoes with butter and milk until silky. Cool 2–3 minutes, then stir in egg yolks and pinch of nutmeg. Season to taste.

In a clean bowl, whisk egg whites until soft peaks form (about 1–2 minutes). Gently fold into mash to retain airy texture.

Transfer mash to a piping bag fitted with a large star tip; pipe rosettes over filling, leaving a small gap at edges.

Bake 20 minutes until duchess potato edges turn golden and lightly crackle. Rest 5 minutes before serving.