Put the lentils in a large mixing bowl and mix with the mustard and freshly ground pepper.
Cut the spring onions into tiny pieces and add a small handful to the lentils.
Heat the oil in a wok, then stir-fry the mushrooms for 2 minutes. Add the garlic and spring onion and the lemon juice and stir for another 3 minutes. Let it cool.
Meanwhile cut the beets into small cubes, add these to the lentils.
Add the stir-fried mushroom and spring onions, then toss well. Season to taste.
Garnish with fresh herbs just before serving.