Loosen the skin from the breast and wing by inserting fingers, gently pushing between the skin and meat. Insert the truffle slices under the loosened skin over the breast and wing. Cover with sage leaves and put the skin back.
Season the chicken with salt and pepper.
Preheat the oven at 350 degrees F.
In a large skillet, on high heat, sear the chicken pieces skin-side down and finish cooking in the oven for 15 minutes.
For the sauce:
Reduce the chicken stock by 1/2 for 30 minutes. Add the Dijon mustard and lemon juice. Slowly add the butter, whisking constantly until emulsified. Season the sauce with salt and pepper, to taste.
Serve the chicken with oven-roasted Jerusalem artichokes.