Pat dry the lamb with paper towels. Cut off some of the tips of the green onions and mince them. You need about 2 teaspoons of minced spring onion tips.
Make 5 deep incisions into the lamb, at the thickest meaty parts, on both sides.
Fill each incision with plenty of sea salt and ground pepper. Add also the minced spring onion tips and the minced fennel fronds. Then press a whole garlic clove into each incision. See Notes below.
Prepare the pan. Chop the remaining spring onions in half and add them to a roasting pan (big enough to fit the leg of lamb). Add also the slices of onions and the fennel fronds.
Add the leg of lamb to the pan. Drizzle with olive oil on top and season with salt and pepper. Sprinkle some salt in the pan as well not only on top of the lamb.
Sprinkle the dry oregano, and pour the wine into the pan.
Marinate. Let lamb sit at room temperature to marinate for 1 hour. Or you can refrigerate and marinate the meat for a couple of hours or overnight. Don't forget to take the pan out of the fridge 1 hour before cooking.
Preheat oven to 230°C / 445°F.
Roast for 15 minutes then drop the heat to 140°C / 285°F.
Slow roast for 2 ½ to 3 hours. Depending on how strong is your oven.
Rest the meat for 30 minutes then cut and serve.