Heat olive oil in a large skillet over medium-high heat.
Add lamb cubes in batches and brown on all sides, about 5 minutes per batch. Transfer browned lamb to the slow cooker.
In the same skillet, add chopped onion and cook until softened, about 3 minutes.
Add minced garlic and cook for 1 minute until fragrant.
Transfer onion and garlic to the slow cooker.
Add carrots, celery, potatoes, beef broth, red wine, tomato paste, dried thyme, dried rosemary, bay leaves, salt, and black pepper to the slow cooker.
Sprinkle 1/4 cup flour over the ingredients in the slow cooker and stir gently to combine.
Cover and cook on low for 6 hours, or until lamb is tender and vegetables are cooked through.
About 30 minutes before serving, prepare the dumplings: In a medium bowl, whisk together 1 1/2 cups flour, baking powder, and salt.
Cut in cold butter using a pastry cutter or fingers until mixture resembles coarse crumbs.
Gradually stir in milk until just combined; do not overmix.
Remove bay leaves from the stew.
Drop spoonfuls of dumpling batter evenly over the surface of the stew in the slow cooker.
Cover and cook on high for 25-30 minutes, or until dumplings are puffed and cooked through.
Sprinkle chopped fresh parsley over the stew before serving.