Wash the potatoes. Cook in boiling water for about 15 minutes. Then rinse with cold water, peel and leave to rest. Wash and quarter the tomatoes and remove the seeds. Cut the quarter in half again.
Crumble or chop the cheese very finely. Whisk cheese, milk, eggs and mustard. Season with salt and pepper. Cut potatoes into slices. Layer potato slices and tomato pieces in a greased casserole dish.
Pour egg milk over it. Cook in the preheated oven (electric cooker: 200 °C/ gas: level 3) for 30-35 minutes. Wash the thyme, pluck the leaves from the stalks and chop. Sprinkle the casserole and serve.