1. The day before, soak the chickpeas in cold water (24 hours).
2. Drain and rinse the chickpeas, then place them in a large pot. Cover with 2 liters of cold water and bring to a gentle boil.
3. Let simmer for 2 hours, then add salt.
4. 15 minutes before the chickpeas are done cooking, heat the oil and butter in a frying pan. Fry the slice of bread, drain, and set aside.
5. Sauté the onions in the frying pan, then add the tomatoes. Add the drained chickpeas and moisten with a few spoonfuls of cooking water.
6. Crush the garlic and fried bread slice with the hard-boiled egg yolk.
7. Dilute with a little water and add this mixture to the chickpeas along with the chopped egg white.
8. Cover and simmer for 30 minutes over low heat.