To cook vegetables in milk, each time you finish cooking a vegetable, pour the contents of the saucepan (milk + vegetables) into a colander placed on a salad bowl, then pour the milk back into the saucepan for the next vegetable. It's quicker and easier than removing the vegetables with a skimmer.
As with all gratins, the quantities given are a guide only, so don't hesitate to adapt them to your own taste, or to what you have on hand.
For a more golden gratin top, sprinkle with an additional 80g of grated cheese, ideally Cantal, before putting in the oven.