Pat the chicken thighs dry and season all over with salt and crushed peppercorns.
In a large frying pan over medium-high heat, heat olive oil. Add the chicken and sear on both sides until deeply golden and almost cooked through (about 5–6 minutes each side). Remove and set aside.
Lower heat to medium and add butter to the pan. Once melted, stir in shallot and garlic and sauté until soft and fragrant.
Take the pan off the heat, pour in the Cognac and let it sizzle. Flambé carefully only if it’s safe to do so. Return to the heat and scrape up the caramelised bits on the base of the pan with a wooden spoon.
Add chicken stock, Dijon mustard, crushed peppercorns, green peppercorns, and thyme sprigs. Bring to a gentle boil, then lower to a simmer for 2–3 minutes. Pour in cream and reduce until thicker.
Return the chicken to the sauce along with any juices. Simmer uncovered until the sauce thickens and coats the back of a spoon (about 2 minutes). Discard thyme stems.
Stir through lemon juice and parsley, taste for seasoning.
