Cut turkey breast across grain into 1/2" slices. Put between sheets of waxed paper and pound smooth and flat (1/4" or less). Dust each slice with flour, salt and pepper. Melt butter and oil in heavy skillet until golden. Add turkey, mushrooms, onions and 1/2 of the lemon juice. Cook turkey 3 minutes on one side; push vegetables aside and turn turkey. Add remaining lemon juice. Cover and cook 2 to 3 minutes until just done. Remove meat to serving platter, stir-fry vegetables 1 or 2 minutes longer, and spoon over turkey. Serve immediately. Serves 4.