Heat 2 tablespoons of the olive oil in a large heavy saucepan. Add the onion and cook over moderately low heat, stirring, until translucent, about 8 minutes. Drain the artichokes well, add them to the saucepan and season with salt and pepper. Reduce the heat to low and cook, stirring, until just tender, about 12 minutes. Add the peas, fava beans and 2 cups of water. Cover partially and cook over low heat until the vegetables are tender, about 6 minutes.