Pecan-Crusted Chicken with Maple-Squash Mash

Preheat oven to 450 degrees F. Toss butternut squash with 1/2 tablespoon of the oil in a shallow roasting pan. Roast 22 minutes, tossing several times, until lightly browned. Meanwhile, pulse pecans, bread, thyme, 1/4 teaspoon of the salt, and cayenne in a food processor until nuts are finely ground. Transfer to a sheet of waxed paper. In a small bowl, whisk egg whites and cornstarch. When squash is tender, transfer to a medium bowl and mash with maple syrup, butter, and the remaining 1/2 teaspoon salt, until fairly smooth; cover with foil. Keep oven on. Dip chicken breasts first in egg-white mixture, then in pecan crumbs to coat. Heat the remaining 1 1/2 tablespoons oil in a large ovenproof nonstick skillet over medium-high heat; add chicken and brown 2 minutes per side, or until lightly browned. Place skillet in oven and roast 5 minutes, until cooked through and golden. Spoon Maple-Squash Mash onto plates; top with chicken. Serve with horseradish sauce.