Preheat the oven to 425 degrees.
In a large bowl, combine the cauliflower, leeks, garlic, olive oil and salt. Toss to coat.
Transfer the vegetables to a rimmed sheet pan, then remove the garlic cloves and wrap them loosely in a small piece of foil. Place the foil packet back on the sheet pan and roast 20-25 minutes, until the cauliflower is nicely browned and can be easily pierced with a fork.
In a large Dutch oven, melt the butter over medium heat. Add the shallot and sauté until it is fragrant and translucent, about 5 minutes. Add the roasted vegetables, vegetable broth, water and 1/2 teaspoon Herbes de Provence. Increase the heat to high, bringing the soup to a boil, then lower the heat and simmer 5-10 minutes, allowing the flavors to combine.
Remove from heat and allow the soup to cool enough to safely handle.Transfer to a blender and puree on high.
Transfer the soup back to the Dutch oven over low heat. Stir in the cream and remaining 1/2 teaspoon Herbes de Provence. Heat to serving temperature.
Garnish with finely chopped chives.