Broccoli and Cauliflower Soup

This recipe was so easy and so good! I got it from the Eating the Greek Way cookbook. Here it is:

Broccoli and Cauliflower Soup

3 T. olive oil
1/2 a large onion, sliced
1 head broccoli, cut into flowerets
1 head cauliflower, cut into flowerets
4 cups water or vegetable broth
salt
pepper
red pepper
1/2 cup yogurt or half and half or milk

Heat oil in large saucepan. Saute onion until soft, about 10 minutes. Add broccoli and cauliflower and saute for five minutes. Add water and bring to a boil. Let simmer for 10 minutes. Puree mix in blender. (Leave vent open so contents don't explode, but cover vent with clean dish cloth.) Return to saucepan. Add salt, pepper, and red pepper to taste. Stir in yogurt. Serve with crusty bread and a salad.