1. Pre-heat the oven to 180 °C.
2. Remove and discard the skin from the snoek, then pull the fish into small pieces, removing all bones. Now, using your fingertips, very carefully check through the shredded snoek two or three times for any smaller bones you might have missed. Place the fish in a big mixing bowl.
3. Coarsely grate the onion on the biggest teeth of your grater, then add the onion pulp and juices to the mixing bowl.
4. Add the grated cheese and parsley and season to taste with salt and black pepper.
5. In a separate bowl, whisk together the eggs, milk and cream. Now add the mayonnaise and whisk again for a minute, or until the mixture is smooth and slightly bubbly.
6. Pour the egg mixture into the bowl containing the snoek and mix very well to combine.
7. Generously grease a deep pie or flan dish with oil or butter. Pour the mixture into the dish and smooth the top with the back of a spoon.
8. Bake at 180 °C for 35-45 minutes, or until golden, puffed and cooked right through. While the pie is still hot, run the tip of a knife around the edges of the dish, to loosen.
9. To make the topping, whisk together the mayonnaise, lemon zest and lemon juice, and season to taste with salt and black pepper.