Trim fat from duck. Rinse, pat dry, then stuff cavity with paper towels to absorb liquid. Place duck in roasting pan and refrigerate uncovered overnight.
Preheat oven to 325°. Remove paper towels, then using your fingers, gently loosen the duck's skin. Pierce duck through skin with a paring knife in 10-12 places and insert garlic slices into incisions. Slide mushrooms beneath skin. Season duck liberally, inside and out, with salt and pepper. Roast for 1 hour and 45 minutes.
Meanwhile, press clove into onion and place onion in a medium saucepan with bay leaf, stock, lentils and a pinch of salt. Bring to a boil over high heat, then reduce heat to medium and simmer, skimming occasionally, until lentils are tender, 30-35 minutes. Discard onion and bay leaf. Set lentils aside.
Remove duck from oven; cool for 10 minutes. Separate chard leaves and stems, then coarsely chop. Heat oil in a skillet over medium heat. Cook chard stems, stirring, for 3 minutes, then add leaves and sage, adjust seasoning, and cook for 3 minutes. Stir in lentils. Carve duck into 4-6 pieces and serve with lentils.