Heat oil in your soup pot over medium heat before adding diced onion, carrots, and celery. Stir to cook everything evenly for a few minutes, or until the onions become translucent.
Bring everything to a boil before adding the lentils, bringing back to a boil, then lowering to a low simmer. Cook for 15 minutes or until the lentils are fully cooked. Stir in spinach and taste to see if the soup needs any additional salt or pepper.