Place olive oil in a large saucepan and warm over medium heat. Add onion and cook, stirring frequently, until it starts to soften, about 5 minutes. Stir in garlic, cook for 2 minutes, and add lamb. Cook until it starts to brown, about 5 minutes, and add carrots and celery. Stir in tomatoes, herbs, cannellini beans, and stock. Bbring to a boil. Reduce heat to medium-low and simmer for 1 hour, until beans are tender. Add spinach and olives and cook until spinach wilts, about 2 minutes. Season with salt and pepper, ladle into warmed bowls, and garnish with feta cheese.