Remove stem ends and dark green stalk of the leeks. Cut in half lengthwise, then in 1/4-inch thick slices. Transfer to a bowl, cover with cold water and swish leeks around to remove any dirt hiding in between the layers. Give dirt a second to settle to the bottom, then lift leeks out of the water and transfer to a plate or cutting board.

Heat oil in a large stock pot over medium heat. Add leeks and season with salt and pepper. Add cauliflower, turmeric, and curry powder, and stir to coat. Season with more salt and pepper and cook until cauliflower is tender, and starts to brown, about 8 minutes.

Add stock and pecans and bring to a simmer. Cover and cook until cauliflower is soft, about 15 minutes. Fill a blender 2/3 full with soup. Place a clean kitchen towel that you don't mind getting dirty (or doubled over paper towel) over the top of the blender. Holding towel down with one hand, turn blender onto its lowest setting. Slowly increase the speed of the blender until soup is smooth and creamy. Repeat with remaining soup, then pour back into the pot and return to the heat.

Bring soup to a simmer adding more stock or water if too thick. Taste and add more salt and pepper if necessary. Ladle into bowls, garnish with roasted red pepper and serve.