LIGHTLY DREDGE THE CHICKEN in the seasoned flour, patting to remove any excess. Brush 1 side of each breast generously with mustard. Pat the pecans into the mustard. Turn and repeat on the other side. Place the chicken in a flat dish, cover, and refrigerate for 20 minutes. In a large heavy skillet, heat the butter and oil just until the butter foams. Add the chicken breasts and saute them over medium-high heat until the pecans are lightly browned, about 3 minutes. Turn and cook the second side for an additional 2 minutes. Remove at once.