Trim the leg of lamb to remove excess fat.

Place the lamb in a nonaluminum pan or a large, heavy-duty plastic bag.

Pour the buttermilk over the lamb, ensuring it's submerged or regularly turned.

Refrigerate for 3 to 4 days, turning the lamb often if not fully covered by the buttermilk.

Preheat oven to 325°F (160°C).

Remove the lamb from the buttermilk and discard the buttermilk.

Dry the meat thoroughly with paper towels.

Rub the lamb with salt.

Place the lamb on a rack in a roasting pan.

Pour water into the bottom of the roasting pan.

Insert a meat thermometer into the thickest part of the roast, avoiding bone contact.

Roast until the internal temperature reaches 160°F (71°C) for medium.

Remove the lamb from the oven and let it rest.

Deglaze the roasting pan by adding a small amount of water and scraping up the browned bits.

Strain the drippings into a saucepan.

In a separate bowl, blend the flour with a little cold water to form a slurry.

Whisk the flour slurry into the strained drippings.

Cook over medium heat, stirring constantly, until the gravy thickens.

Remove from heat and stir in the sour cream, salt, and pepper.

Slice the roasted lamb against the grain.

Serve the sliced lamb with the sour cream gravy.