This method works for small chickens and hens, of up to 3 lbs. Cornish 2-lb hens work well. Season with kosher salt inside and out. Don't be sparing with the salt, use a bit more than you think you need. Take room-temperature unsalted butter, which should be soft in a "mayonnaise-like" consistency. Butter the birds under the skin.
Carefully slice the black truffle in slightly thicker slices than you would use for shaving. Layer the black truffle slices under the skin. It should be abundant, generous.
Now stuff the cavity with a cut onion or two. Truss the birds with cooking string. Arrange in a roasting pan. Now smear more of the softened butter on the outside of the skin. Add some carrots and sliced shallots to the pan, along with a couple of cloves of garlic still in their skins. As the birds roast, the butter will melt and coat the entire outside.
Depending on how big your birds are, you may want to have one per person. It's nice for each person to get a perfect little bird to dig into. A larger one can be carved then served in traditional service.
Roast them for 30 minutes at 450 degrees in a convection oven. The high heat will crisp the skin, and the convection will move the hot air around in the oven in a way that cooks the birds evenly. After 30 minutes at 450, reduce the temperature to 300 degrees, and let them cook for 30 minutes more. Now let them rest, in the oven, with the heat turned completely off, for 30 more minutes.
This method results in perfectly cooked, moist dark and white meat, nicely crisp skin, and a rested, juicy bird. You can use the roasting liquid and fat, along with the giblets, to make a pan sauce, or just go with the natural jus that comes off each bird.
Take the uneven end of the truffle, chop finely, and mix well with more softened butter. Use this delicious elixir to finish your pan sauce, or simply to smear on toast.