Start by making the bechamel sauce. In a saucepan, melt butter over medium high heat. Once the butter has melted, sift in the cake flour, creating the roux. Mix around vigorously to ensure that the flour is evenly distributed and cooks evenly. Keep mixing around until the flour is completely cooked and it turns a light golden brown.

Gradually add in the cold milk. Add the milk bit by bit, stirring after each pour to prevent lumps. It will get pretty solid and look like mashed potatoes, but trust the process and keep pouring and mixing.

Once the sauce is smooth and thick, add the heavy cream, bouillon powder, salt, and pepper. Mix to fully incorporate and set aside.

Now, prepare the add-in ingredients. In a medium sized shallow pan, heat some neutral oil. The amount of oil you use depends on the amount of ingredients you will add. For a basic gratin, about a teaspoon of oil will be enough to cook the onions. Sprinkle the onions with a little bit of salt and pepper, and saute until they start to turn translucent and start to lightly brown. Remove from the pan and set aside.

Cook your pasta very al dente according to the package. The pasta will continue to cook in the oven, so we don’t want it to get too soft. Drain the pasta and set aside.

Now we can start to assemble the components of the gratin. Simply add the onions, pasta, and other ingredients into the saucepan with the sauce. Mix to combine evenly. Depending on the ingredients you used or the size of the saucepan, it may be easier to combine all of these in a large bowl.

Coat the inside of a deep baking dish with unsalted butter. Scoop the gratin mixture into the dish, and top with mozzarella cheese. Bake at 430°F/220°C for 15-20 minutes or until the cheese starts to brown and is bubbly. You can top with some parsley to garnish, and the gratin is done! 完成です！