Place a medium or large pot on stove over medium heat. Add cooking oil and chopped onion. Cook for 5-6 minutes, until onion is tender.
Add chopped carrots, celery, potatoes, lentils, diced tomatoes, vegetable broth, water, salt, pepper, garlic powder, cumin, and thyme. Turn heat up to high and bring soup to a boil. Turn heat back down to medium-low and lightly simmer for 25-30 minutes, or until lentils and potatoes are both tender.
Run a potato masher through the soup (or use a fork) to lightly mash some of it to thicken. Chop fresh spinach or kale. Add spinach/kale and lemon juice to the soup and allow the greens to wilt. Enjoy!
Store leftover soup in the fridge for up to 5 days, or freeze in large containers or pint-size mason jars for individual servings up to 3 months.