Clean mushrooms, removing any dirt or debris that may be stuck to them. Rough chop them up.
Rough chop the onion as well. Since you will be blending this soup up, you will not need to worry about how precise it's chopped.
Melt ½ cup or 8 tablespoons butter to sizzle in a large soup pot over medium-high heat.
Add the sliced mushrooms and chopped onions.
Lightly sauté them until the mushrooms slightly soften and the onions look translucent.
Stir in the flour until all the white streaks are gone, making a paste.
Add the chicken broth a little at a time, stirring as you go until all the broth has been added.
Simmer until the mushrooms are soft all the way through. They will be soft but not rubbery.
Remove the pot from the heat for a moment. If you are wanting chunks of mushrooms or large pieces of mushrooms in your finished soup, remove them from the pot and set them aside.
Blend the soup until smooth using an immersion blender, bullet blender, or food processor.
Return the blended soup to the pot with any mushroom chunks or pieces you set aside.
Turn the stovetop temperature a bit lower than medium heat.
Add the slightly warmed half-and-half garlic powder and white pepper.
Add salt to taste.
Don't allow the soup to come to a boil again. Just heat until hot.
Serve a warm bowl of soup with crusty bread!
Store soup in an airtight container in the fridge!