Fill a medium pot with water and add the bay leaf, juice of 1/2 lemon and both lemon halves. Bring to a boil.
In another pot, bring 1 quart water to a boil. Salt the water and add the bread, stirring and mashing until a thick porridge forms, about 5 minutes. Remove the pot from the heat and stir in the garlic, cilantro and EVOO. In a small bowl, beat the egg, if using, then stir into the bread porridge.
Add the prawns to the boiling lemon water and season with salt. Cook until pink and firm, 3 to 4 minutes; drain.
Serve the stew in shallow bowls and top each with 5 prawns.