In a large bowl, toss the mushrooms and cauliflower in about 1/4 cup of olive oil, and scatter over a baking pan. Sprinkle salt and pepper over the vegetables. Roast for about 20 minutes or until fork-tender and golden.
In a medium saucepan, heat the remaining 1 tablespoon of olive oil over medium low heat and add the leeks. Sauté until wilted, about 3 minutes. Add the roasted mushrooms and cauliflower, as well as the chicken stock. Bring to a low boil, then working in two batches, blend the soup in a blender, taking care not to overfill the blender as hot liquid could spill. Return the puréed soup into the saucepan, and return to a simmer. Season to taste with salt and pepper, if necessary. Stir in the crème fraiche, and ladle into bowls. Garnish with chives and serve immediately.