Soak dried beans in water, 8 hours to overnight. Drain, rinse, and set aside.

Heat olive oil in a large stockpot over low heat. Cook and stir onion in hot oil until just beginning to soften, about 4 minutes. Stir in garlic and cook for 1 minute. Increase heat to medium. Add lamb; cook for 3 to 4 minutes. Stir in carrots and celery; cook for 1 minute. Stir in tomatoes, then season with parsley, thyme, oregano, pepper, and salt. Add beans and broth. Bring to a boil, cook for 5 minutes, then reduce heat to medium-low and simmer, covered, until beans are tender, about 1 hour 30 minutes.

Heat a large saucepan over medium heat. Sauté spinach in the hot saucepan until wilted. This will only take a short time, and you will not need to add more liquid to the pan.

Ladle soup into bowls. Top each serving with wilted spinach and feta cheese.