I put the ingredients in order of being used to make things easier for you.
Cook the pasta in a separate pan ordering to the box directions.
In a large skillet, melt butter and cook onion until tender. Then, add ham and peas and heat through.
In a mixing bowl, whisk corn starch and milk together and add to the ham and peas. On medium low heat heat the milk through until it starts to thicken.
One the milk mixture has thickened add the Parmesan cheese, mix well. Salt and pepper to taste.
Add the cooked noodles to the creamy sauce and top with additional Parmesan cheese and serve!