Heat the olive oil in a large pot or Dutch oven over medium heat. Add the diced onion and sauté for about 5 minutes, or until softened and translucent.
Add the minced garlic, diced carrots, and diced celery to the pot. Cook for an additional 3-4 minutes until the vegetables begin to soften.
Stir in the sliced mushrooms and continue cooking for 5 minutes until the mushrooms are tender and have released their moisture.
Add the rinsed lentils, vegetable broth, diced tomatoes (with their juice), dried thyme, ground cumin, and paprika. Season with salt and pepper to taste. Stir everything together until well combined.
Bring the stew to a boil, then reduce the heat to low. Cover and let it simmer for 30 minutes, or until the lentils are tender.
After the lentils are cooked, add the chopped kale or spinach and stir until wilted, about 2-3 minutes.
Taste the stew and adjust seasoning if necessary. Serve hot, garnished with fresh parsley if desired.
For added flavor, consider adding a splash of soy sauce or balsamic vinegar before serving.