Cook the onions with the olive oil on a big pot. As soon as they are tender, add the cauliflower. I like to stir fry the vegetables before adding water. Cook the cauliflower with the onions for a few minutes and add about 5 cups of water. Add them one at a time, so you can choose how runny your soup will be.
As soon as the cauliflower is cooked, turn the heat down. Puree the soup on a blender or using a mixer. I like some texture, but you can make it really smooth if you want.
Check the seasoning. Add the peas and the butter and stir well. I love fresh peas, so sweet and delicious! So just heat them up and turn the heat off.