Heat a large Dutch oven over medium-high heat. Add 2 teaspoons oil to pan; swirl to coat. Add onion, thyme, and rosemary; sauté for 5 minutes, stirring occasionally. Place onion mixture in a large bowl.
Place flour in a shallow dish. Sprinkle lamb evenly with 1/2 teaspoon salt and 1/2 teaspoon pepper. Dredge the lamb in flour, and shake off excess. Return pan to medium-high heat. Add 1 tablespoon oil to pan; swirl to coat. Add half of the lamb mixture to pan; sauté for 6 minutes, turning to brown on all sides. Add browned lamb to onion mixture. Repeat the procedure with the remaining lamb and the remaining 1 tablespoon oil.
Add beer to pan; bring to a boil, scraping pan to loosen browned bits. Cook until reduced to 1 cup (about 5 minutes). Return onion mixture and lamb to pan. Stir in the tomato paste; cook 30 seconds.
Add broth and bay leaf; bring to a boil. Cover, reduce heat, and simmer for 1 hour and 15 minutes, stirring occasionally.
Uncover and stir in potato, carrot, and turnips. Simmer, uncovered, for 1 1/2 hours or until meat and vegetables are tender. Stir in remaining 1/2 teaspoon salt, the remaining 1/4 teaspoon pepper, and 1 tablespoon mustard. Ladle about 1 cup stew into each of 7 bowls; sprinkle evenly with parsley, if using.