Cut the onion. It is not important what shape or size it is cut into, but the smaller it is cut, the faster it will cook.
Melt the butter (or ghee for Whole30, or olive oil for vegan) in a skillet and sauté the onion until it is translucent and has just begun to color.
Add all of the ingredients to a blender and blend until smooth. If you are using a high-powered blender like a Vitamix, you can continue to blend on high power until you can see steam rising from the lid. If you do not have a high-powered blender, pour the pureed soup into a large pot and cook over medium heat until warmed through.