Sweet Potato Soup

I tried this soup for dinner, it was delicious!

1/2 cup chopped onion
2 garlic cloves, chopped
2 medium orange sweet potatoes, peeled and cubed
1 1/2 cups squash, cubed (I used a Magda Hybrid, but any mild squash will do)
2 sprigs fresh rosemary
6 cups vegetable broth
dash black pepper
1/2 cup sour cream
cheddar cheese, optional

Heat a large saucepan over medium high heat. add approx 1 Tbsp oil to the pan. Add onions and garlic, saute for about 2 minutes. Add yams, squash, rosemary, black pepper and vegetable broth. Bring to a boil, reduce heat and simmer covered for approx 40 minutes. Remove the rosemary sprigs. Using an immersion blender (or a regular blender), blend until soup is thick and smooth. Add sour cream and stir until incorporated. Serve in bowls and top with a little cheddar cheese if desired.