Peel and dice the potatoes into 1-inch cubes. Rinse the lentils thoroughly under cold water and pick out any debris. Chop the onion, dice the carrots and celery, and mince the garlic.

Add the diced potatoes, rinsed lentils, chopped onion, diced carrots, diced celery, minced garlic, undrained diced tomatoes, vegetable broth, dried thyme, smoked paprika, bay leaves, black pepper, and salt to a 6-quart (or larger) slow cooker. Stir well to combine all ingredients.

Cover the slow cooker and cook on low for 7-8 hours, or on high for 3.5-4 hours, until the lentils are tender and the potatoes are easily pierced with a fork.

Once cooked, remove and discard the bay leaves. Stir in the fresh baby spinach (or chopped kale) until it wilts, about 5-10 minutes.

Taste and adjust seasoning with additional salt and pepper if needed. For an extra bright flavor, stir in fresh lemon juice before serving. Ladle into bowls and garnish with fresh chopped parsley, if desired.