Wash and chop the vegetables. Dice the onion, carrots, and celery. Slice the mushrooms and mince the garlic.
Rinse the lentils under cold water. In a separate pot, bring water to a boil and add the lentils. Cook for about 10 minutes until slightly tender. Drain and set aside.
In a large pot, heat the olive oil over medium heat. Add the diced onion and sauté for about 5 minutes until translucent. Add minced garlic and cook for another minute. Then add carrots, celery, and sliced mushrooms. Cook for 5-7 minutes until softened.
Stir in the pre-cooked lentils, dried thyme, oregano, smoked paprika, black pepper, and salt. Pour in the vegetable broth and add the can of diced tomatoes. Stir everything together.
Bring the mixture to a boil, then reduce heat to low and cover. Let it simmer for 25-30 minutes. Stir occasionally. After simmering, add chopped kale or spinach and cook for an additional 5 minutes. Stir in lemon juice before serving.