In a large pot over medium heat, toss the diced onion with the olive oil and salt, saute until the onion is just transparent. Add the celery and carrots, saute until both have just softened.

Add the garlic, crushed tomatoes, cumin, oregano, sage, and pepper. Stir constantly for one minute, then add the lentils.

Add 3 empty tomato can-fulls of water, bring to a rolling boil, then reduce the heat and let simmer for 20 to 30 minutes, or until the lentils are fully cooked.

Add the vinegar. Add the spinach one handful at a time, stirring until it's wilted. Serve immediately.