Melt the butter in a pot over medium heat. Add the sliced onions and sauté until they soften and turn a very light golden color.

Add the broth to the onions, then add all the remaining ingredients. Cover the pot.

Once it comes to a boil, reduce the heat and simmer until everything is tender and the garlic is very soft.

Remove from heat. Blend the soup using a blender or immersion blender, then strain.