Season the chicken with salt, pepper and the poultry seasoning.In a large skillet, heat the EVOO, 1 turn of the pan, over medium-high heat. Add the chicken and cook, turning once, until golden and cooked through, about 5 minutes. Transfer to a plate and cover with foil to keep warm.
Add 3 tablespoons butter to the skillet and lower the heat to medium. Add the mushrooms and cook until softened, about 5 minutes. Add the shallot and cook for 2 minutes; season with salt and pepper. Sprinkle in the flour and cook, stirring, for 1 minute. Whisk in the chicken stock and bring to a simmer. Lower the heat to low and stir in the cream, tarragon and mustard. Cook until the gravy thickens, 1 to 2 minutes.
Lightly brush the toast with the remaining 1 tablespoon butter and set the chicken cutlets on top. Cover with the gravy and watercress.
