Step 1: Place a large pot on the stove on medium heat and add the butter to the pot. Once the butter is melted, add the chopped onion and cook until the onion is translucent.
Step 2: Add the remaining ingredients to the pot and bring everything to a boil.
Step 3: Turn down the heat and allow the ingredients to simmer for ~30 minutes (until they are soft).
Step 4: Let the soup cool for a few minutes and then use your immersion blender to blend well, until smooth.