Heat the olive oil in a large pot or Dutch oven. When hot, add the onion, carrots, and celery. Season generously with salt and fresh ground pepper, and cook for 4-5 minutes until the vegetables are tender and the onions are translucent.
Add the bell peppers and cook for about 6 minutes until tender.
Stir in the sweet potatoes and vegetable broth. Cover the pot with a lid and bring the soup to a boil. Once boiling, remove the lid and lower to a simmer. Stir in the marjoram and simmer for 20-25 minutes until the sweet potatoes are very tender.
Time to puree! You can do this with an immersion blender, or transfer the soup to a regular blender and puree in batches.
Return the soup to the pot. Taste and season with a little extra salt and pepper if necessary. (I added a pinch of cayenne. Spice lovers, I recommend following my lead.)
Ladle soup into bowls and garnish with avocado, cilantro and crumbled crackers (if you like).