Preheat oven to 200°C (180°C fan-forced).
Place the fennel, cauliflower & garlic together in a large mixing bowl with two tablespoons of olive oil & toss together until well coated.
Divide the fennel mixture between two baking trays & roast in preheated oven for 45 minutes or until golden. Remove from oven & peel off garlic skins.
Heat remaining olive oil in a large saucepan over a low heat. Add leeks, cover & cook for 5 minutes or until soft.
Add the roasted vegetables, stock & potatoes to the saucepan. Increase the heat to medium, cover & cook for 30 minutes.
Using a stick blender, puree soup until smooth.
Chop reserved fennel fronds & add to soup with sour cream, truffle salt & pepper, stirring until
well combined.
To make the croutons, lightly grill the baguette slices on one side.
Meanwhile, combine the fennel seeds, Gruyere & thyme together in a small mixing bowl.
Turn the toasts over & evenly divide the cheese mixture between them.
Place back under the grill until the cheese has melted.