To make soup, in a medium-sized pot over medium-high heat cook butter and oil until bubbling. Add onion and garlic and sauté for 5 minutes or until onion is soft. Add mushrooms and cook for 4 minutes or until cooked. Add flour, mix well with a wooden spoon then cook over a medium heat for 3 minutes. Slowly add hot stock, mixing well to ensure there are no lumps. Bring to a simmer for 6 minutes.
Remove from heat and process with an immersion blender (or in a blender) until smooth. Return to pot and add first measure of crème fraiche, salt and pepper to your taste. Mix to combine and set aside.
To make walnut crème fraiche, in a small bowl combine all ingredients.
To make croûtes, drizzle ½ tsp oil on each side of baguette slices. Grill baguette slices on one side then turn, sprinkle with gruyère and grill until golden-brown and bubbling.
Prior to serving, reheat soup and add sherry (if using). Divide soup between 4 bowls. Top each croûte with 1 tbsp walnut crème fraiche, Drizzle your favourite balsamic over each soup.