Finely grind toasted walnuts in a food processor. Transfer the nuts to medium bowl; mix in breadcrumbs and chopped sage. Drizzle melted butted and 3 tablespoons syrup over nut mixture; toss to moisten evenly. Season to taste with salt.
Boil broth, sage sprigs, thyme sprigs, and remaining 2 tablespoons syrup in medium saucepan over high heat until reduced to 1 cup, about 12 minutes. ( Can be made 8 hours ahead. Cover nut mixture and keep at room temperature. Cover and chill reduced broth.).
Preheat oven to 450°F Heat oil in very large ovenproof skillet over medium-high heat. Sprinkle chicken breasts with salt and pepper, then place flat side down in skillet. Cook until chicken is golden brown on bottom only, about 4 minutes. Remove skillet from heat.
Turn chicken over. Mound walnut mixture atop golden side of each chicken breast, dividing equally and pressing to adhere. Transfer skillet to oven and bake until topping is crisp and golden brown and chicken is cooked through, about 8 minutes.
Transfer chicken to platter; tent with aluminum foil to keep warm. Add reduced broth to same skillet, stirring to scrape up any browned bits on bottom of skillet. Boil over high heat until broth is reduced to ¾ cup, about 3 minutes. Reduce heat to low. Add chilled butter 1 piece at a time, whisking until melted before adding more. Season sauce to taste with salt and pepper. Strain sauce through fine-meshed strainer.
Saute pancetta in heavy medium skillet over medium heat until crisp and golden brown, about 20 minutes.
Meanwhile, cook potatoes in large pot of boiling lightly salted water until tender, about 15 minutes. Drain potatoes, reserving 1 cup cooking liquid. Transfer potatoes to large bowl. Add butter and ¾ cup reserved cooking liquid. Mash, thinning with additional cooking liquid if desired. Stir in pancetta and up to 3 tablespoons pan drippings. Season potatoes to taste with salt and pepper.
Mound potatoes on plates. Place chicken atop potatoes. Drizzle sauce around potatoes and serve.