In a large pot, heat oil over medium heat. Sauté onion, garlic, asparagus and broccoli until fragrant (about 5 minutes). Cover with broth, add potatoes and bring to a boil. Reduce heat to medium-low and simmer gently until vegetables are tender, 15 to 20 minutes. Remove pot from heat and set aside to let cool slightly. Carefully transfer soup to a blender (in batches) and purée until smooth. Ladle soup into bowls, garnish with sour cream and serve.