Heat 2 tablespoons of oil in a large sauté pan over medium-high heat. Sear the chicken thighs on both sides and transfer to a paper towel lined plate. In the same pan, fry the onion and garlic until the onion is translucent and starting to turn golden in color, 5-8 minutes. Add the flour and stir to coat the onion and garlic. Add the white wine, stirring constantly to ensure the flour combines with the liquid and there are no lumps. Leave the wine to simmer until it has reduced to just a couple of tablespoons.
Add the chicken back to the pan and pour in the stock. Toss in the bay leaves. Cover and simmer gently over low heat for 20 minutes, turning the chicken once halfway through cooking.
While the chicken is cooking, heat the remaining 1 tablespoon of olive oil in a separate skillet over medium-high heat. Add the mushrooms and fry until they are just starting to soften, 3-5 minutes. Set aside.
Combine the dry and wet mustards in a small bowl, stirring until you have a smooth mustard.
Once the chicken is cooked, remove the thighs from the pan and add the cream and mustard to the sauce. Ensure the heat is on low so the mixture does not boil. Taste the sauce and add any salt or pepper you would like. Add the chicken and mushrooms to the sauce and garnish with some chopped tarragon. Best enjoyed immediately.