Heat the grill to a high setting. Halve the romaine lettuces lengthways. Place on a baking tray cut side up. Drizzle with 1/2 tbsp olive oil and season. Slide under the grill and cook for 4-5 mins till charred.
Finely slice the leek and celery. Heat a pan. When warm, drizzle in 1 tbsp olive oil. Add the sliced veg. Cook for 4 mins till softened. Peel and grate the garlic. Add to the pan and cook for 1 min.
Peel and chop the potato into cubes. Add it to the pan. Crumble in the stock cube and pour in the boiling water. Pop on a lid and simmer gently for 10 mins.
Finely shred the charred lettuces. Add three halves to the pan. Cook for 2 mins. Peel, core and grate the apple. Add to the pan. Roughly chop the mint leaves and add most.
Blitz the soup in the pan with a stick blender till smooth. Or transfer to a food processor. Zest and juice half the lime into the soup. Taste and season, adding more lime juice as necessary. Stir through the remaining shredded lettuce. Scatter over the remaining chopped mint.