Slice the onion and trim the shimeji mushrooms, separating them.

Roll out 2 pie sheets slightly larger than the baking dish. Line the dish with the pie sheets, folding excess inward and piercing the bottom with a fork. Chill for 30 minutes, then bake at 200°C for 15 minutes.

Boil macaroni according to package instructions, then drain.

Heat salad oil in a pan and sauté onion and shimeji mushrooms until softened. Remove from the pan.

In the same pan, melt butter and stir in flour over low heat for 1.5 minutes. Gradually add milk, stirring over medium heat until thickened. Add macaroni and sautéed vegetables, seasoning with salt and pepper.