Make the dressing in a medium-sized bowl by combining the buttermilk, soured cream, mayonnaise, lemon juice and 50g of the blue cheese. Season and stir through the chives. Chill until ready to serve.
When ready to serve, pour half the dressing over the lettuce wedges and crumble over the remaining blue cheese and a scattering of walnuts. Serve the rest of the dressing on the side.