Using meat mallet or rolling pin, flatten chicken pieces between sheets of waxed paper to scant 1/2-inch thickness. Season with salt and pepper. Melt 1 tablespoon butter with 1 tablespoon olive oil in heavy large skillet over medium-high heat. Saute chicken until light brown, about 2 minutes per side. Transfer to plate. Add remaining 1 tablespoon butter to same skillet and melt over medium heat. Add leeks and mushrooms and cook until just beginning to color, stirring occasionally, about 5 minutes. Add wine, cream and chopped tarragon and simmer until leek mixture thickens slightly, about 5 minutes. Return chicken and any accumulated juices to skillet and simmer until chicken is just cooked through and leeks are tender, about 5 minutes longer. Adjust seasonings. Arrange chicken on serving platter. Spoon sauce over. Garnish with tarragon sprigs and serve. chick 17