1. Peel the onions and cut them in very thin slices. Clean the mushrooms and cut them into small pieces. Heat up three spoonfuls of oil in a frying pen; add the onions and the mushrooms. Put the lid on and sauté them during 20 minutes. Cook with the lid on and stir from time to time.
2. Clean the liver with cold water, strain with kitchen roll. Take off all the external skin and any large veins. Chop the liver into cubes adding some salt and pepper.
3. Heat up the remaining two spoonfuls of oil and, when it’s very hot, put the liver and fry it over a high flame.
4. Pour the liver on the first frying pen and cook it during some minutes, always stirring. Then, put them on a platter and garnish with lemon slices.