For a six lb., bone in, leg of lamb, preheat oven to 350' F. Line your roasting pan with aluminum foil, shiny side up. Make a compound butter by combining the herbs and butter together. Place the leg fat-side up on a cooking rack in the roasting pan. Slice shallow, diagonal slits through the fat, to the meat..Rub the compound butter into the slits. Cook to an internal temp of 135' F (about 90 minutes). Remove the roast from the oven to a serving platter. Deglaze the pan with your wine. Pour off liquid into a saucepan. Bring to a simmer.
Combine 1 tbs. cornstarch with 1/4 cup water. Mix to make a smooth slurry. Add that to the pan, whisking as you drizzle it in. When thickened, spread over your roast. Serve with Potato Anna, steak fries, or Hasselback potatoes, and a fresh green salad with Greek salad dressing. You really need to get onboard with using a good meat thermometer. It removes the guesswork from cooking everything from a leg of lamb, to a smoked turkey. Here's a link; https://www.amazon.com/ThermoPro-Wir.../dp/B01GE77QT0
There are many, less expensive versions online.
Hope that helps.
Seeeeya; hief Longwind of the North