Heat the butter in a pot or large saucepan over medium heat. Add the carrots and onions and cook for about 5 minutes, until softened. Add the 3 tablespoons flour and the thyme, stirring so that the vegetables are evenly coated. Slowly add the stock, whisking to prevent lumps from forming. Bring to a gentle simmer. Season the chicken thighs with salt and black pepper and add to the pot, submerging them in the stock. Poach the chicken for about 8 minutes, until just cooked through. Remove to a cutting board to rest. Combine the remaining 2⁄3 cup flour with the baking powder, rosemary, 1⁄4 teaspoon salt, and lots of black pepper. Add the milk and gently stir until the dough just comes together. Form loosely into 8 dumplings and drop them directly into the soup. Cover the pot and cook over low heat for 10 minutes, until the dumplings have firmed up. Shred the reserved chicken. Add to the pot, along with the peas, stirring carefully so as not to break up the dumplings. Heat through for 1 minute before serving.