Preheat the oven to 190°C (170°C fan) and position a shelf in the center.
Pat the chicken dry and season with salt and pepper. Lightly coat with flour, shaking off any excess.
Heat the olive oil and butter in a large ovenproof skillet over medium-high heat.
Sear the chicken for 5–6 minutes per side until golden brown. Remove and set aside.
In the same pan, cook the shallots for 3–4 minutes until softened. Add the garlic and cook for 1 minute.
Pour in the white wine and scrape up any browned bits from the bottom of the pan. Simmer until reduced by half.
Add the chicken stock and tarragon. Bring to a gentle simmer.
Return the chicken to the pan, cover, and cook for 10–15 minutes until fully cooked.
Remove the chicken temporarily. Stir in the cream and Dijon mustard.
Add the halved grapes and simmer gently for 2–3 minutes. Do not boil.
Return the chicken to the sauce and spoon the sauce over the top.
Garnish with fresh parsley and serve immediately.