Rinse the lentils under cold water and set aside.
In a pan, heat the olive oil over medium heat. Add the diced onion, carrots, and celery, sautéing for about 5 minutes until softened.
Add the minced garlic and stir for another minute until fragrant.
Transfer the sautéed vegetables to the crockpot.
Add the rinsed lentils, diced tomatoes (with their juice), vegetable broth, ground cumin, dried thyme, bay leaf, salt, and pepper to the crockpot.
Stir to combine all the ingredients well.
Cover the crockpot and cook on low for 6-8 hours or on high for 3-4 hours.
About 30 minutes before serving, stir in the chopped kale or spinach (if using).
Remove the bay leaf before serving. Adjust seasoning with salt and pepper if needed.