Pound 2 garlic cloves and salt into a smooth paste with a mortar and pestle. Add the cumin, paprika, juice from the lemon, and chopped cilantro and mix until slightly emulsified. This is the chermoula.
Reserve a little of the chermoula for cooking and rub the rest of it over the monkfish. Cover and leave to marinate in a cool place for 1 hour.
Par-boil the potatoes for about 10 minutes and then cut them in-half. Thinly slice the remaining garlic cloves. Heat 3-4 tbsp of the olive oil in a heavy pan and stir in the garlic. When the garlic just begins to color, add the tomatoes and cook until just softened.
Add the peppers to the tomatoes and garlic, together with the remaining chermoula, and add salt and pepper to taste.
Spread the potatoes over the base of a shallow pan or deep, ridged frying pan. Spoon 3/4 of the tomato and pepper mixture over and place the marinated fish chunks on top with the chermoula you reserved in step #2.
Spoon the rest of the tomato and pepper mixture over the fish and add the olives. Drizzle a little extra olive oil over the dish and pour in the water.
Heat until simmering. Cover and cook over medium heat for 15 minutes, or until the fish is cooked through.