Heat the oil in a large saucepan, over medium heat. Add the onion, garlic, and Italian seasoning and cook for 2-3 minutes.
Add the remaining vegetables (celery, carrots, red pepper) and cook, covered, over medium-low heat for around 20 minutes. Continually check and stir the vegetables to ensure that the vegetables don't burn. They should be tender when pierced with a fork.
Add the crushed tomatoes, broth, white beans, sun-dried tomatoes (if using) and basil. Stir until combined. Bring to a boil, reduce the heat and allow to simmer for an additional 20 minutes.
Allow to cool slightly and then transfer to a blender and blend until smooth. You may need to do it in batches. Taste and season with salt and pepper, to taste (skip the salt if making for baby).
Allow to fully cool before pouring the sauce into containers or mason jars for storing.