Clean and roughly chop the mushrooms and toss with the juice of 1/2 lemon. Melt half the butter with half the oil in a large sauté pan. Chop the scallions and garlic and add to the pan with the mushrooms, half the tarragon, and salt and pepper to taste. Sauté over medium heat until most of the liquid has evaporated. Transfer the the contents of the pan to a bowl and reserve for later use.

Slightly flatten the chicken breasts by pounding with a rolling pin between sheets of waxed paper. Season on both sides with salt, pepper, and paprika and dredge lightly in flour. Heat the remaining butter and oil in the pan and sauté the chicken until brown and cooked through, about two minutes on each side. Remove the chicken to a warm serving platter.

Deglaze the pan with the wine and reduce over high heat for several minutes. Add the reserved mushroom and scallion mixture, the cream, and the mustard and continue cooking, stirring continuously, until thickened to a coating consistency. Pour the sauce over the chicken and top with the remaining tarragon.