1 Cook potatoes in a pan of salted, boiling water over medium-high heat, until just tender, about 15 minutes. Drain, cool slightly and cut into pieces. Set aside.
2 Slice the white and light green parts of the leek into rings. Parboil the carrots in a pan of salted water for about 4 minutes, add leeks and zucchini and cook for an additional 1 to 2 minutes. Drain immediately.
3 Preheat oven to 400º F. Grease a casserole dish with butter.
4 Melt 2 tablespoon butter in a small saucepan over medium heat. Add onion and saute until soft and translucent. Stir in flour, stirring constantly. Gradually add milk and vegetable stock, simmer for 5 minutes over low heat. stirring occasionally. Add ham and herbs, stirring to incorporate. Season with salt, pepper and nutmeg. Add 1/2 of the cheese, stirring until just melted. Let cool slightly.
5 Melt the remaining tablespoon of butter in a skillet. Pan fry the potatoes for several minutes, add the vegetables, stirring gently to coat with butter. Spoon potatoes and vegetable into prepared casserole dish.
6 Whisk egg yolks and add them to the sauce. Pour sauce over the vegetables and sprinkle with the remaining cheese. Bake for 20 to 25 minutes, or until top is golden brown. Let cool for 5 minutes. Serve.