In a pot, heat the butter/Ghee over medium heat. Add the finely chopped onion and sauté. When translucent, throw in the chopped mushrooms and cook for a while. Pour in the broth, add chopped asparagus and cook until soft. Use an immersion mixer to mix everything. In a cup combine a bit of water, flour, and heavy cream. Pour it into the soup while blending all ingredients together and cook for a while. Season with salt, pepper, and finely chopped fresh parsley. Serve with croutons.