How to Make It
In a small skillet, toast the peppercorns and fennel seeds over moderate heat until fragrant, about 2 minutes. Transfer the spices to a large bowl and add the water, fennel bulb and fronds, balsamic vinegar, honey and salt. Whisk to dissolve the salt. Add the duck breasts to the bowl; cover and refrigerate overnight.
Preheat the oven to 325°. In a small saucepan, melt the butter. Add the onion and cook over moderately high heat, stirring, until browned. Add the honey and cook over moderate heat until the onion starts to caramelize, 2 minutes. Add the wine and simmer until reduced by half, 4 minutes. Add the stock, peppercorns, thyme and tomato and cook over moderately high heat until the sauce has thickened enough to coat the back of a spoon, about 10 minutes. Strain the sauce into a bowl and season with salt and pepper.
Remove the duck from the marinade and pat dry; season with salt and pepper. In a large, ovenproof skillet, arrange the duck skin side down and cook over moderate heat until browned, about 12 minutes. Turn the duck breasts skin side up and transfer the skillet to the oven; cook until medium-rare, about 7 minutes. Transfer the duck to a cutting board and let rest for 5 minutes. Slice the breasts crosswise 1/4 inch thick. Spoon the Celery Root and Potato Puree onto plates, top with the duck and the Pinot Noir Sauce.