1) Halve, slice and wash the leeks well and drain. Remove the woodiest part of the green leaves.
2) Heat the oil in a frying pan over a medium heat. Add the leeks and cook until softened but not brown, about 5 minutes.
3) Add the peas and continue cooking until the peas are thawed and bright green, about 3 minutes.
4) Transfer the peas and leeks to a blender along with the sour cream, salt, parsley and lemon. Blend on low until smooth.
5) Serve the dip in a bowl garnished with the slices of spring onion.