Make the croutons: Pre heat the oven to 350ºF. In a large bowl, place 3-4 tablespoons of oil, Dijon mustard, salt, and pepper. Mix and add the pieces of bread into the bowl. Stir everything well, until all the pieces of bread are covered by the mixture of oil and mustard. Place them on a tray and put them in the oven for 5 minutes, flip them with a spatula, and bake for an additional 5 minutes or until golden and crunchy. Take them out of the oven and let cool.
Make the soup: In a saucepan over medium heat, combine olive oil, onion, and garlic and sauté until the vegetables are golden brown. Add cauliflower and water, cover, reduce heat and leave for 15 minutes or until cauliflower is cooked. When the cauliflower is cooked, blend everything with Dijon mustard. Use an immersion blender or regular blender. Add salt and freshly ground pepper.
Serve warm, topped with croutons, and a drizzle of olive oil.