1. Cook the pasta : bring water to a boil, stir in salt (1 tsp) and add the pasta. Cook the pasta according to the package directions. Drain well and set aside.
2. Heat 1 tbsp oil in a large frying-pan over medium heat. Add the onion and cook until wilted.
3. Add the butter in the pan and let it melt completely. Then add the flour to the mixture, stirring to combine with the onions.
4. Pour the milk gradually. Mix well each time. Once all the milk is incorporated, add shrimps and ham, mix well. Then add also the seasoning salt, sugar, heavy whipping cream and parmesan cheese, stirring frequently. Cook for a few minutes.
5. Finally, add the cooked pasta. Mix well.
6. Divide the mixture into each baking dish. Top with shredded cheese (of your choice), as much as you want, over the gratin.
7. Pre-heat the oven to 200℃. Bake the gratin for 15 min. Cover with alminium and bake 5 min more.