1. For turkey: Grind dried porcini in processor until almost reduced to powder, with some larger bits remaining, about 4 minutes. Transfer to sieve. Shake powdered porcini into bowl; mix in 2 tablespoons butter. Reserve larger bits in sieve. Rinse turkey inside and out; pat dry with paper towels. Place turkey on small rack set in large roasting pan. Rub porcini butter over outside of turkey; sprinkle with salt and pepper. Bring porcini bits and chicken broth to boil in small saucepan. Remove from heat; reserve for basting. (Turkey and porcini broth can be made 1 day ahead. Cover separately; chill.)
2. Set rack at lowest position in oven and preheat to 325 degrees F. If stuffing turkey, spoon stuffing loosely into main cavity and neck cavity. Tuck wing tips under; tie legs together loosely. Roast turkey 1 hour 15 minutes, covering loosely with foil if browning quickly. Toss onion, carrots, garlic, and oil in bowl to coat; scatter in pan around turkey. Baste with 1 cup porcini broth. Continue to roast turkey until thermometer inserted into thickest part of thigh registers 175 degrees F, basting with 3/4 cup porcini broth and pan juices every 45 minutes, about 2 hours 30 minutes for unstuffed and 3 hours for stuffed. Transfer turkey to platter; let stand 30 minutes (internal temperature will increase 5 to 10 degrees).
3. For gravy: Place roasting pan over 2 burners. Add turkey stock and any remaining porcini broth and bring to boil, scraping up browned bits. Strain turkey broth into heavy large saucepan; spoon off fat.
4. Mix 4 tablespoons butter and flour in bowl to form paste. Melt 3 tablespoons butter in large skillet over medium heat. Add mushrooms. Cover; cook until mushrooms release liquids, stirring occasionally, about 8 minutes. Add Madeira; boil until wine evaporates, about 2 minutes. Add mushroom mixture to turkey broth; bring to boil. Whisk in flour paste. Reduce heat to medium; simmer until gravy thickens, whisking occasionally, about 5 minutes. Season with salt and pepper.
5. Serve turkey with gravy.

Yield: 10 servings