Heat olive oil in a large pot over medium heat.

Cook sausage until browned, about 8-10 minutes. Transfer sausage to a bowl with a slotted spoon.

Add onions, carrots, zucchini, and celery to the pot and cook until vegetables are soft and translucent, stirring often, about 7 minutes.

Add the basil, italian herb seasoning, salt, pepper and minced garlic and cook for another 2 minutes, while stirring to ensure the garlic does not burn.

Then, stir in the lentils.

Add the chicken broth to the pot. Bring the soup to a boil, and then reduce the heat to medium and allow the soup to simmer while stirring occasionally for about 20 minutes, or until the lentils are soft.

Add the sausage back to the soup and stir to incorporate.

Let the soup simmer for another 10-12 minutes.

Stir in the spinach and cook until it is wilted, about 3 minutes.