Prepare the chicken: Pat the chicken breasts dry with kitchen paper. This is crucial for a good sear. Season them evenly on both sides with the salt, pepper, and garlic powder. The chicken should look lightly dusted with the seasoning.

Sear the chicken: Heat the olive oil in a large frying pan over a medium-high heat. You want the oil to shimmer, but not smoke. Carefully place the chicken in the pan. You should hear a satisfying sizzle — if it doesn't sizzle, the pan isn't hot enough. Cook for 4-5 minutes per side, until the underside is a deep, golden brown and releases easily from the pan. The chicken will not be cooked through at this stage. Remove the chicken to a plate and set aside.

Sweat the aromatics: Reduce the heat to medium. Add the chopped onion to the pan. It will sizzle in the remaining oil and juices. Cook for 4-5 minutes, stirring occasionally, until the onion is soft and translucent. You'll notice a sweet, savoury aroma filling your kitchen. Add the minced garlic and dried thyme, and cook for a further minute until the garlic is fragrant, but not burnt.

Make the roux: Sprinkle the flour over the softened onions. Stir constantly with a wooden spoon for 1-2 minutes. The mixture will look thick and pasty, and the flour will turn a light golden colour. This step cooks out the raw flour taste, ensuring a smooth, not lumpy, gravy.

Build the gravy: Slowly pour in the warm chicken stock, whisking continuously to incorporate the flour mixture. The liquid will immediately start to thicken. Bring the mixture to a gentle simmer; you'll see small bubbles breaking on the surface and the gravy will have a smooth, glossy sheen.

Finish the gravy: Reduce the heat to low. Stir in the double cream and Dijon mustard. The sauce will turn a beautiful pale ivory colour and become wonderfully velvety. Taste and adjust seasoning with a pinch more salt and pepper if needed. The gravy should be rich and savoury, with a subtle tang from the mustard.

Return the chicken: Place the seared chicken breasts back into the pan, nestling them into the creamy gravy. Spoon a little of the sauce over the top of each breast. The gravy should come about halfway up the sides of the chicken.

Simmer to perfection: Cover the pan with a lid and let it cook on a low simmer for 20-25 minutes. The chicken will gently poach in the sauce, becoming incredibly tender. You'll know it's done when the chicken is firm to the touch and the internal temperature reaches 74°C. The gravy will have thickened slightly more and will be coating the back of a spoon.

Rest and serve: Remove the pan from the heat and let it rest for 5 minutes. This is not just a suggestion; it allows the juices to redistribute within the chicken, ensuring every bite is moist. Garnish generously with fresh parsley before serving.