Preheat oven to 350°. In a Dutch oven heat 1 tbsp olive oil over medium heat. Sprinkle lamb with salt and pepper. Add to the pot and sear for 1-2 minutes, or until lightly browned, on top and bottom. Remove to a plate.
Add 1 tbsp olive oil to the pot. Add onion, garlic, celery, sage, and rosemary. Cook for 3-4 minutes until vegetables start to soften.
Add ½ tsp salt, ¼ tsp pepper, flour, and tomato paste and mix until combined. Mix in Guinness.
Add carrots, potatoes, parsnips, cabbage, and lamb. Bring to a simmer then cover and remove from heat. Transfer to oven for 1 hour, or until meat and vegetables are tender
Toast and butter bread, if desired, just before serving. Divide stew into bowls and top with parsley.