You can add onion, celery and carrots pretty much to any soup (may be not cream of mushroom or crab bisque) but any soup: chicken, vegetable, beef, lentil, bean.
Favorite and super easy- put in those three finely diced with 1-2 Tbs olive oil, on medium heat and cover until soft, about 5 min. Season with salt & pepper. Then add 1.5 cups red lentils and 4.5 cups broth or water plus 1 bay leaf. Simmer 20 min. Add 2 tbs. lemon juice as you take off the heat. Sometimes I add a diced potato with the veggies. Yummy!