Heat olive oil in a large pot over low heat. Add the onion, sauté for 1 minute, and add the garlic. Stir and cook for 30 seconds. Add the carrots, bell peppers, and fennel. Cook for 5 minutes, stirring occasionally. Add the vegetable broth, cover, and simmer until the veggies are tender, 20-25 minutes. Season the soup with salt, black pepper, and red pepper flakes.
Transfer the soup to a blender and puree until perfectly smooth. Divide between 4 bowls. Garnish generously with fresh parsley, slices of red radish, sunflower seeds, and red radish sprouts. Serve immediately.