Melt butter in a saucepan over high heat. Add garlic and onion. Cook, stirring, for 5 minutes or until onion has softened. Add stock. Bring to the boil. Reduce heat to medium. Add peas. Cook for 4 to 5 minutes or until heated through. Stir in lettuce. Serve.
Variation: Add 1 tablespoon shredded fresh mint leaves with lettuce.