Cut 8 small florets from the cauliflower head and reserve for garnish. Coarsely chop the rest of the cauliflower head, including the healthy green leaves. Clean the leeks and cut the white part only into 1-inch pieces (about 1 cup). Place the cauliflower, leeks, and stock in a medium pot, cover, and place over medium-high heat. Bring to a boil, decrease heat to medium-low, and cook for 25 min. or until the vegetables are tender.
Working in 3 or 4 batches, transfer the soup to a blender or food processor and puree until smooth. (Note: Be very cautious with the hot liquids. Do not fill the blender of food processor more than half full).
Transfer the first pureed batch to a clean pot and repeat the process. In the last batch, add the miso and salt, and then puree.
Cut the reserved cauliflower florets into thin slices lengthwise. Toss with olive oil, curry powder, and a pinch of salt. Cook the cauliflower in a skillet over high heat until lightly golden, stirring occasionally.
Reheat the soup and divide into soup bowls. Garnish with the sautéed cauliflower.
Sprinkle a tiny pinch of salt over each soup bowl and serve.