Prepare the Beans: Rinse the 15-bean mix under cold water. If time allows, soak the beans overnight in a large bowl of water. Drain and rinse before cooking.

Optional Sauté: Heat olive oil in a skillet over medium heat. Sauté the diced onion, carrots, celery, and garlic for 3-5 minutes until fragrant and slightly softened. This step enhances flavor but can be skipped if desired.

Add Ingredients to Slow Cooker: In a 6-quart slow cooker, combine the rinsed beans, sautéed vegetables (if using), diced tomatoes, vegetable broth, smoked paprika, cumin, chili powder, thyme, bay leaf, and a pinch of salt and pepper.

Cook the Soup: Cover and cook on low for 8-10 hours or on high for 4-6 hours. Check occasionally to ensure the beans are tender. Add water or broth if the soup becomes too thick.

Add Greens (Optional): During the last 20-30 minutes of cooking, stir in fresh spinach or kale if desired. Allow the greens to wilt.

Adjust Seasoning and Serve: Remove the bay leaf. Taste the soup and adjust seasoning with additional salt and pepper as needed. Serve hot with a squeeze of lemon for brightness.