Preheat and prepare: Preheat your oven to 200°C (400°F) and lightly grease a standard 12-hole muffin tin. Place a large frying pan or saucepan over medium-high heat.

Sauté the aromatics: Add the olive oil to the pan. Once it shimmers and you can smell the warm oil, add the diced onion, carrot, and celery. Cook for 5-6 minutes, stirring occasionally, until the onions turn translucent and the carrots soften slightly — you'll notice a sweet, earthy aroma filling your kitchen.

Cook the chicken: Add the cubed chicken and minced garlic to the pan. Cook for 4-5 minutes, stirring frequently, until the chicken is no longer pink on the outside and has a light golden colour. The sizzling sound will slow down as the chicken releases its juices.

Build the sauce: Sprinkle the flour over the chicken and vegetables, then stir constantly for 1 minute. You'll see the flour absorb the fat and juices, forming a thick paste. Gradually pour in the chicken stock while whisking, then add the milk. Bring the mixture to a gentle simmer — you'll see small bubbles breaking the surface — and cook for 3-4 minutes until the sauce thickens enough to coat the back of a spoon.

Finish the filling: Stir in the frozen peas and dried thyme. Season generously with salt and pepper. Remove the pan from the heat and let it cool for 5 minutes. The filling should look thick and glossy, with visible chunks of tender chicken and colourful vegetables.

Assemble the muffin cups: On a lightly floured surface, roll out each puff pastry sheet slightly to about 3mm thickness. Cut each sheet into 6 equal squares (you'll have 12 squares total). Gently press each square into a muffin hole, letting the corners hang over the edges. Spoon about 3 tablespoons of filling into each pastry-lined cup — the filling should be level with the top of the tin.

Top and bake: Fold the overhanging pastry corners toward the centre, pinching them together lightly to create a rustic top. Brush the tops with beaten egg for a golden, shiny finish. Bake for 20-25 minutes, rotating the tin halfway through. The muffin cups are ready when the pastry is puffed and deep golden brown — you'll hear a satisfying crackle when you tap the pastry.

Rest and serve: Let the muffin cups cool in the tin for 5 minutes. This resting time allows the filling to set slightly, so the cups hold their shape when you remove them. Use a small knife to loosen the edges, then lift them out. The bottoms should be crispy, and the tops flaky.