In a large, heavy saucepan, melt 2 tablespoons butter over low heat. Add the onion and cook, stirring until softened, about 5 minutes. Increase the heat to high, add 2 tablespoons butter and the mushrooms and cook, stirring constantly, for 3 minutes. Add the chicken broth and bring to a boil. Reserve 2 tablespoons thyme leaves; tie the remaining sprigs together with kitchen string and add to the pot. Lower the heat and simmer for 20 minutes.
Meanwhile, toast the bread. In a small saucepan, melt the remaining 2 tablespoons butter; stir in the garlic and reserved thyme leaves. Brush the garlic-thyme butter on the toasted bread.
Discard the thyme bundle and stir 1 cup cream into the soup. Using an immersion or standing blender, puree the soup until smooth. Heat through and season with salt and pepper.
Divide the soup among 4 bowls. Drizzle with the remaining 1/4 cup cream and serve with the toasts.
