Take lamb out of the fridge 30 minutes before you intend on putting it in the oven. This allows the meat to relax, which adds to the tenderness of the meat.
Preheat oven to 400F.
Place leg of lamb in a roasting tin and score the fat on the lamb. Make several deep incisions across the top and push small, 2-inch pieces of fresh rosemary into each incision with a slice of garlic. Rub a small amount of olive oil over the skin and season with sea salt and freshly ground black pepper.
Roast the lamb for 20 minutes in the hot oven, then turn the heat down to 350F and cook for another hour or so for rare, 1 1/4 hours for medium and 1 1/2 hours for well-done.
When the lamb is cooked, remove it from the tin, tent it with foil and allow it to rest for 15 minutes.
While the leg of lamb is resting, pop the roasting pan over medium heat. If using red wine (I highly recommend it!), add it to the pan and whisk it into the pan juices. Let simmer 3 minutes to allow the alcohol to evaporate.
Add the stock, whole garlic cloves and chopped fresh rosemary and bring to a boil. Then whisk in the butter and flour to thicken slightly. Pour in the cream if using (it adds a lovely richness). Taste and allow to bubble up until the flavor is concentrated enough. Season with sea salt and freshly ground black pepper and pour through a sieve into your gravy bowl.
For the Zippy Fresh Mint Sauce:
Place 5 tablespoons of finely chopped fresh mint, 3 tablespoons of white wine vinegar, 2 teaspoons of sugar and 1 tablespoon of hot water in a bowl. Whisk well and boom you have a mint sauce!
Serve Potato Cakes alongside with Mini Irish Coffee Shots for dessert.