Cut off the woody end of the asparagus, then cut each in half.
Finely chop the spring onions.
Heat the olive oil in a large frying pan.
Add the mushrooms, then gently stir over medium heat for about 5 minutes, then add the vinegar.
Add the spring onion and asparagus pieces, then cook over low heat and under a lid for another 5 minutes.
Stir in the lentils and mustard, turn up the heat and stir for a couple of minutes until the sauce has reduced.
Stir in the lemon juice and herbs, then season to taste.
Serve warm.