Add onions and celery and sautee for a few minutes until they start to soften.
Add carrots, sauce, water, and lentils.
Cover and bring to a gentle boil.
Reduce heat to simmer. Cook for 35 to 45 minutes. Taste and adjust seasonings as needed.
Notes
The taste of your soup will vary depending on which spaghetti sauce you used. You may want to add garlic, Worcestershire sauce, salt and pepper, or your favorite hot sauce.