How to Make It
Preheat the oven to 450°. In a large bowl, separate the onion rings and toss with 2 tablespoons of the oil, the thyme and sage. Spread the onions out on a large baking sheet and roast for about 20 minutes, tossing once, until tender and golden. Leave the oven on.
Meanwhile, score the fat on the duck in a crosshatch pattern; season the duck with salt and pepper. Heat a large ovenproof skillet. Add the duck, skin side down, and cook over moderate heat until the skin is golden brown, about 7 minutes. Turn the duck and roast in the oven for about 6 minutes for medium rare. Transfer the duck to a cutting board and let rest for 5 minutes.
Add the pancetta to the skillet and cook over moderate heat until browned and crisp, about 5 minutes; drain on paper towels. Pour off all but 2 tablespoons of the fat from the skillet. Add the garlic to the skillet and cook over high heat until fragrant but not browned, about 1 minute. Add the demiglace and vinegar and bring to a boil, scraping up any browned bits. Remove the skillet from the heat; whisk in the walnut oil and the remaining 2 tablespoons of olive oil and season the dressing with salt and pepper.
Arrange the baby greens and roasted onions on 4 large plates and drizzle with some of the dressing. Thinly slice the duck breasts crosswise and arrange over the salads. Drizzle the remaining dressing on top, garnish with the pancetta and serve.
Demiglace is available at specialty food stores and from D'Artagnan (800-327-8246).