Preheat the oven to 350°F (180°C).

Slice the potatoes and zucchini thinly (use a mandoline if you have one).

Slice the onion and mushrooms thinly.

In a bowl, mix the half-and-half, milk, cornstarch, salt, pepper, and mustard.

Cut the chicken breast fillets into small pieces.

Sauté the onions in a skillet with a little oil or butter for 5 minutes.

In a baking dish, layer potatoes, mushrooms, onions, and zucchini, repeating until all ingredients are used.

Sprinkle the shredded Swiss cheese on top and bake for 45 to 55 minutes.

Insert a fork into the gratin to check if it’s cooked through.