In a large ovenproof skillet cook pears in hot butter over medium heat for 8 to 10 minutes or until golden brown and tender, stirring frequently. Sprinkle with coarsely ground pepper; remove from skillet and set aside.

Trim excess fat from duck (do not remove skin). Score the skin in a diamond pattern. Season duck breasts with salt and 1/4 teaspoon pepper.

In the same skillet cook duck breasts, skin side down, over medium heat for 5 minutes. Turn and cook about 5 minutes more or until browned. Drain, reserving 2 tablespoons fat. Place pan in a 350 degree F oven; roast, uncovered, for 25 to 30 minutes or until an instant-read thermometer inserted into the breast registers 155 degrees F. Remove duck from pan; cover and let stand for 10 minutes (the internal temperature will rise 5 degrees F during standing time).

For vinaigrette, in a screw-top jar combine reserved duck fat, the vinegar, mustard, and thyme. Cover and shake well to combine.

Slice duck breasts. To serve, arrange spinach on individual serving plates. Top with pears, duck breast slices, and toasted walnuts. Drizzle vinaigrette over top. Makes 4 servings.