Pre-heat your oven to 375°F (190°C) with a rack in the middle.

Heat up a large oven-safe frying pan (or skillet) over medium heat with the butter. When the butter is sizzling, place the chicken pieces in, skin-side down. Cook for about 10 minutes, without touching, until the chicken skin is golden brown and the chicken is ¾ done; just firm. Flip the chicken pieces and cook for 10 more minutes. Transfer the chicken to a plate and cover with foil. Keep the pan hot and do not rinse it (keep the chicken drippings in).

Add the onion and garlic to the pan. Cook for about 10 minutes, stirring occasionally, until the onion is lightly caramelized. Nestle the chicken pieces back into the pan, on top of the onion. Sprinkle with the chopped tarragon. Pour the chicken stock and white wine into the pan. Bring to a simmer and transfer the pan into the oven for 30 minutes (covered).

Take the pan out of the oven, back onto the stove top at medium heat. Transfer the chicken to a plate and cover with foil again. Stir the cream into the pan. Bring to a simmer and keep stirring until the sauce thickens (this could take 7-8 minutes). Place the chicken thighs back into the pan and cook for 5 more minutes.

For serving, sprinkle chicken pieces with fresh tarragon leaves.