Pat the chicken dry and season with salt and black pepper.
Heat olive oil and butter in a large frying pan over medium heat.
Cook the chicken for 5–6 minutes per side until golden brown. Remove and set aside.
In the same pan, cook the onion or shallots until soft and translucent.
Add the garlic and cook for 30 seconds until fragrant.
Pour in the chicken stock and scrape up any browned bits from the pan.
Stir in Dijon mustard, wholegrain mustard, and thyme.
Add the double cream and stir until smooth.
Return the chicken to the pan and simmer gently for 10–12 minutes until cooked through.
Stir in lemon juice and parsley.
Taste and adjust seasoning if needed.
Serve hot with your favorite side dishes.