1.Melt butter in a saucepan on medium. Cook onion and garlic for 2-3 minutes, until softened but not coloured.
2.Add lettuce, potato and stock, and bring to boil. Reduce heat and simmer for 10-15 minutes, until potato is tender.
3.Using a hand mixer or food processor, blend soup until smooth.
4.Stir in dill and lemon rind. Season to taste.