Place eggs in a pan of salted water. Bring the water to the boil and let eggs cook in boiling water until they are hard boiled, approximately 10 to 15 minutes. Drain eggs and let cool.
Meanwhile, heat the oil in a frypan and cook the onion over medium heat until soft; about 5 minutes. Add the mushrooms and cook until they've lost most of their liquid.
Once cool to the touch, peel the eggs. Cut in half longways and gently scoop out the yolks. Mash the yolks and combine with the cumin and mayonnaise. Stir until smooth then stir in the mushroom mixture. Spoon or pipe yolk mixture back into the egg whites.