Heat oil in a large pot over medium heat. Add onion, carrots, celery, and a pinch of salt and black pepper. Cook until vegetables are tender, about 4 minutes. Add bell peppers and cook until soft, about 6 minutes. Add sweet potato and broth. Season with salt and black pepper, cover and bring to a boil. Reduce heat to a simmer and add marjoram. Simmer until potato is tender, about 20 minutes.
Let soup cool slightly, then transfer to a blender in batches and puree until smooth. If needed, thin soup with water. Adjust seasoning with additional salt and black pepper, if necessary. Return soup to pot to keep warm until serving. Garnish with croutons, avocado, cilantro, and/or sriracha, if desired.