Soak the night before the chickpeas in water together with the soda. The day after wash and place in boiling water. Let it boil a while to a uncovered pot and remove the foam created while boiling. When no longer foaming add the chopped onions, salt and pepper. Close the lid and cook for 60 minutes. Once finished, add the olive oil and the corn flour which you have firstly been dissolved in a little cold water. Leave to simmer for 5 to 10 minutes.