Combine butter, truffles, thyme, ¾ teaspoon Kosher salt and ½ teaspoon black pepper in a small bowl and mix until combined. Set aside until ready.
Truffle Butter Roast Turkey
Place turkey, breast side up, on a rack set inside a large, shallow roasting pan. Let sit at room temperature for 1 hour.
Towards the end of the resting period, preheat oven to 450°F.
Scoop ⅓ of the truffle butter into a separate bowl and set aside for basting. Pat turkey dry and use your fingers to loosen skin on the breast. Work half of the truffle butter under skin, spreading evenly over both breasts. Smear outside of turkey with remaining half of butter. Season the cavity with salt and pepper. Place the thyme, lemon, onion and garlic in the cavity. Tie legs together with kitchen twine and pour 1 cup water into roasting pan.
Roast turkey until skin is mostly golden brown, about 30 minutes. Reduce oven temperature to 300°F and continue to roast turkey, basting with remaining truffle butter every 30 minutes, until an instant-read thermometer inserted into the thickest part of breast near the neck registers 160°F, 2 ½ to 3 hours. Remove from the oven, tent with aluminum foil and allow to rest for 15 to 20 minutes. Carve and serve.