Heat oil in a saucepan over medium-high heat, add leek and garlic, season to taste and sauté until tender (4-5 minutes). Add half the stock, bring to the boil, add peas and lettuce, and simmer until peas are bright green and just tender (1-2 minutes). Remove 1 cup peas and refrigerate until required.
Process pea mixture along with mint, yoghurt, lemon juice and remaining stock in batches until smooth. Season generously to taste and chill well (30 minutes). Serve topped with reserved peas, lemon rind, pea tendrils, extra mint and yoghurt.