As with all stews, the flavors deepen if chilled overnight and reheated. Also, once chilled, the fat is easily skimmed from the top.
Trim the fat from:
If the lamb is boneless, cut into 1 ½-inch pieces. Sprinkle with:
Heat in a Dutch oven over medium-high heat:
Add the meat in batches and brown on all sides. Transfer the meat as it browns to a platter. Reduce the heat to medium and add:
Brown until golden, then transfer the onions to the platter with a slotted spoon and drain off any fat from the pot. If using, cut the leek into bite-sized pieces. Melt in the pot:
Gradually whisk into the butter:
Cook, stirring, until the flour is golden and fragrant, about 5 minutes. Slowly whisk in:
Increase the heat to high, scraping the bottom to release any browned bits. Let the wine reduce by about half and then return the meat and onions to the pot along with:
Bring to a simmer, cover, and cook until the lamb is fork-tender, about 1 ½ hours. Add to the pot:
Cover and simmer until the vegetables are tender, about 20 minutes longer. Remove the bay leaf and thyme sprigs. If using bone-in cuts, transfer them to a baking sheet. When cool enough to handle, remove any bones, discard any gristly bits, cut into chunks, and return the meat to the pot. Chill the stew overnight, if desired. Skim off any fat from the surface of the stew, return to a faint simmer over low heat (if necessary), and stir in:
Heat through and serve at once, sprinkled with: