In a heavy-bottomed pot, heat 2 tablespoons of the olive oil. When the oil is hot, add the lamb and brown evenly. Remove the lamb from the pot and set aside. Add the leeks and saute until soft. Add the butter and melt. Add the flour to make a roux. Turn the heat down to low, and cook the roux until it is brown, about 15 minutes. Whisk in the beer and stock. Add the tomatoes and the reserved lamb, bring to a simmer, and cover. Simmer, covered, for 1 1/2 hours.
While the lamb mixture is simmering, combine the parsnips, carrots, potatoes and garlic in a large bowl. Toss with remaining olive oil, salt and pepper and rosemary sprigs. Place the vegetables in a roasting pan and place in a 400-degree oven. Roast for 45 minutes, stirring every 20 minutes, until vegetables are tender. Remove the vegetables from the oven and discard the rosemary sprigs. Add the roasted vegetables and the peas to the lamb mixture. Cook for 10 minutes. Add the chopped rosemary. Season with salt and pepper. Add the thinly sliced leeks to a pot of heated oil and fry for 3 minutes, or until crisp. Remove the leeks from the oil and drain on paper towels. Season the leeks with salt. Garnish the stew with the fried leeks.