Add butter to a large pot on medium heat. Add chopped onion once butter is melted.
Sauté onions for 5 minutes, stirring occasionally. Meanwhile, chop the mushrooms.
Add the mushrooms to the pot and increase heat to medium-high. Cook for 10-15 minutes, stirring frequently, until the mushrooms have released their fluid and have shrunk down.
Add the garlic and Dijon mustard to the pot. Stir.
Add the remaining soup ingredients to the pot and reduce heat to medium once it starts to boil. Simmer for 10 minutes, or until mushrooms reach your desired tenderness.
Meanwhile, prepare your toppings. To make the garlic baguette toasts, I just used my toaster and then rubbed the garlic over the toast. You could mass toast the bread slices in the oven if you wish (375F, brush slices w. oil, add to baking sheet, 10-15 min.).
Transfer about half the soup to a separate bowl or pan, and use an immersion blender (or a regular blender) to purée half the soup. Transfer the puréed soup back into the other soup pot.
Top each bowl of soup with whichever toppings you fancy. I like to dip a couple garlic baguette slices in at once and let them soak up the soup. :)