Simmer the lentils with the bay leaf,
one of the onion stuck with the cloves, and the celery in plenty of
water in a covered pan for 40-45 minutes, or until tender.
Drain and discard the bay leaf, onion,
cloves and celery.
Leave to cool.
Stir together the mint and yoghurt and
season with salt and pepper.
Slice the remaining onion into rings.
Stir together the lentils, onion rings
and mushrooms and stir in the yoghurt dressing.
Turn into a serving dish and garnish
with the tomatoes.