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 diced onion and cook for 3 minutes until softened.
Add minced garlic and cook for 1 minute until fragrant.
Transfer onion and garlic to the slow cooker.
Add sliced carrots, celery, and potatoes to the slow cooker.
In a bowl, whisk together beef broth, red wine, tomato paste, dried thyme, dried rosemary, salt, and black pepper.
Pour the liquid mixture over the lamb and vegetables in the slow cooker.
Add bay leaves and sprinkle 1/4 cup flour over the ingredients in the slow cooker. Stir gently to combine.
Cover and cook on low for 6 hours until lamb is tender and vegetables are cooked through.
About 30 minutes before serving, prepare the dumpling dough by combining 1 1/2 cups flour, baking powder, and salt in a mixing bowl.
Add milk and melted butter to the dry ingredients and stir until just combined to form a sticky dough.
Remove bay leaves from the stew and give it a gentle stir.
Drop spoonfuls (about 2 tablespoons each) of dumpling dough onto the surface of the stew, spacing them evenly.
Cover and cook on high for 30 minutes without lifting the lid to allow dumplings to steam and cook through.
Once dumplings are puffed and cooked, serve the stew hot.