Chop the onion. Sauté in the margarine along with the mushrooms until tender. Stir in the curry powder and cook for a few moments longer. Chop the tomato and add to the saucepan, with the salt. Heat through. Add the lemon juice and yoghurt. Stir well and heat until just below boiling point.
Peel and halve the avocados. Place on the rice and fill with the mushroom mixture.