Prepare the Chicken: Pat the chicken breasts thoroughly dry with kitchen paper. This step is crucial for achieving that beautiful sear. Season generously on both sides with salt and freshly ground black pepper. Lightly dust each chicken breast with plain flour, shaking off any excess. You’ll notice the chicken has a fine, even coating, ready for searing.

Sear the Chicken: Heat the olive oil in a large frying pan or skillet over a medium-high heat until it shimmers. Carefully place the chicken breasts in the hot pan, ensuring they don't touch. You should hear a satisfying sizzle as they hit the pan. Sear for 5-7 minutes on one side until deeply golden brown and a lovely crust has formed. Flip the chicken and sear for another 5-7 minutes on the second side until it’s also golden and cooked through. The internal temperature should reach 74°C (165°F). Remove the chicken from the pan and set aside on a plate, loosely tented with foil to rest.

Sauté Aromatics: Reduce the heat to medium. Add the butter to the same pan, allowing it to melt and foam gently. Add the minced shallot and cook for 2-3 minutes until it softens and turns translucent, releasing a sweet aroma. Stir in the minced garlic and dried thyme, cooking for another minute until fragrant, being careful not to let the garlic brown.

Build the Sauce Base: Pour in the chicken stock, scraping up any delicious browned bits (fond) from the bottom of the pan with a wooden spoon – this adds immense depth of flavour. Bring the liquid to a gentle simmer.

Whisk in Flavour: Add the fresh lemon juice and Dijon mustard to the simmering stock. Whisk continuously until the sauce is smooth and slightly thickened. The sauce should have a lovely pale yellow hue and a creamy consistency. If you're adding chilli flakes, stir them in now. Continue to simmer for 3-5 minutes, allowing the sauce to reduce slightly and the flavours to meld beautifully.

Combine and Serve: Return the rested chicken breasts to the pan, nestling them into the zesty lemon mustard sauce. Spoon the sauce over the chicken to coat it thoroughly. Let it warm through for a minute or two. Garnish generously with fresh chopped parsley before serving immediately. The vibrant green of the parsley contrasts beautifully with the golden chicken and creamy sauce.