Turmeric adds color and a depth of flavor in this cauliflower leek soup, while fried leeks provide a textural contrast. Serve with crusty bread.
Heat oil in a small saucepan over medium-high heat. Add 1 cup leeks; cook, stirring often, until golden brown, 6 to 8 minutes. Use a slotted spoon to transfer the fried leeks to a paper-towel-lined plate; set aside.
Transfer the leek oil to a large Dutch oven. Add the remaining leeks; cook over medium heat, stirring often, until tender, about 8 minutes. Stir in cauliflower, coriander, salt, turmeric and pepper. Add broth and bring to a boil over medium-high heat; reduce heat to medium-low and simmer until the cauliflower is tender, about 20 minutes. Remove from heat.
Transfer the cauliflower mixture to a blender (in batches, if necessary). Secure the lid on the blender and remove the center piece to allow steam to escape (use caution when blending hot liquids). Place a clean towel over the opening. Process until smooth, about 45 seconds. (Alternatively, use an immersion blender to puree the soup in the pot until smooth, about 3 minutes.)
Return the soup to the pot; stir in half-and-half and lemon juice. Divide among 6 bowls; top each with 1 tablespoon yogurt and some of the fried leeks. Sprinkle with pepper, if desired.
Refrigerate soup (without garnishes) for up to 5 days or freeze for up to 2 months.