The night before: pat the turkey breast dry. If it came in netting, leave the netting on. Mix the kosher salt and pepper and rub it all over the skin and the underside. Set it on a rack over a sheet pan, uncovered, in the fridge for 12 to 24 hours.

Take the turkey out 45 minutes before roasting. Heat the oven to 325F. Soften the butter and mix it with the minced thyme, rosemary, sage, garlic, and lemon zest.

Loosen the skin from the meat with your fingers, starting at one end, without tearing it. Push the herb butter under the skin and spread it over the meat. Pat the skin dry again. If the roast is not netted, tie it with kitchen twine every 2 inches so it holds an even shape.

Cut the onion into thick slices and lay them in the center of a roasting pan. Set the turkey on the onions, skin side up, and pour 0.75 cup of the broth into the pan.

Roast 75 to 90 minutes, until a thermometer in the thickest part of the roast reads 160F. Start checking at 60 minutes. If the skin browns too fast, tent it loosely with foil.

Move the turkey to a cutting board and let it rest 20 minutes, uncovered. It will climb to 165F as it rests. Do not skip this or the juice ends up on the board.

For the gravy: pour the pan drippings through a strainer into a measuring cup and skim off most of the fat. Add the remaining 0.75 cup of broth to make about 1.5 cups of liquid. Pour into a small saucepan and bring to a simmer.

Stir the cornstarch into 3 tablespoons of cold water until smooth. Whisk it into the simmering liquid and cook 2 minutes until the gravy thickens and coats a spoon. Taste and add pepper; it usually needs no salt.

To carve: snip off the netting or twine. Slice the roast crosswise, against the grain, about 1/2 inch thick. Serve with the gravy.