Melt butter in a large saucepan, add vegetables and cook until soft but not browned.
Add lentils and vegetable stock to saucepan, cover and simmer gently for 35 minutes or until lentils are soft.
Pour into a colander to drain off liquid and then transfer to a large bowl.
For vinaigrette, add shallot, parsley, Extra Virgin Olive Oil and Mustard to food processor. Put on high to blend.
Pour over salad and toss, season with salt and pepper.
Serve either warm or cold
If a food processor is unavailable, dice shallot and parsley fine by hand, pour in Extra Virgin Olive Oil and Mustard and whisk together until blended.