- In a large saucepan over medium heat, soften the leek and onion in the oil. Add the potato and broth. Bring to a boil. Reduce the heat, cover and simmer gently until the potato is tender, about 20 minutes. Add the lettuce. Bring to a boil and cook for 1 minute. Remove from the heat and add the yogurt.
- Purée in a blender. Strain if desired. Let cool. Cover and refrigerate overnight. If the soup is too thick after chilling, add a small amount of chicken broth. Adjust the seasoning before serving (see note).
- With the rack in the middle position, preheat the oven to 180°C (350°F).
- In a bowl, combine the lemon zest, lemon juice, oil, anchovy paste and garlic. Brush both sides of the tortillas with the mixture. Sprinkle with Parmigiano-Reggiano and cut each tortilla into 8 triangles. Season with salt and pepper.
- Arrange the tortillas cheese side up on a parchment-lined baking sheet. Bake until golden, 12 to 15 minutes. Let cool.
- Serve the soup in glasses and accompany with the crackers.