Heat the olive oil in a skillet over medium heat. Add the diced onion and sauté for about 3 minutes until softened.

Add the minced garlic to the skillet and cook for another 1 minute, stirring frequently to prevent burning.

Transfer the sautéed onion and garlic to the slow cooker.

Add the diced carrots, celery, lentils, sweet potato, canned diced tomatoes (with juices), and vegetable broth to the slow cooker.

Stir in the ground cumin, smoked paprika, dried thyme, ground coriander, turmeric, cayenne pepper (if using), and a pinch of salt and black pepper.

Cover the slow cooker with the lid and cook on LOW for 7-8 hours or on HIGH for 3-4 hours, until the lentils and vegetables are tender.

Once cooked, stir in the fresh spinach and let it wilt in the hot soup for 5 minutes.

Taste the soup and adjust seasoning with additional salt, pepper, or a squeeze of fresh lemon juice, if desired.

Ladle the soup into bowls and serve warm. Store leftovers in an airtight container in the refrigerator for up to 4 days.