"You can use any kind of vegetables for this casserole, whatever you have in your fridge - zucchini, broccoli, tomatoes. Don't dice the vegetables too small, all the vegetables should be about the same size."
Place potatoes into a large pot and cover with salted water; bring to a boil. Reduce heat to medium-low and simmer until they are almost tender, about 15 minutes. Drain in a colander.
Preheat oven to 350 degrees F (175 degrees C). Grease a large baking dish.
Melt 2 tablespoons butter in a large skillet while potatoes are cooking and cook onion until soft and translucent, about 5 minutes. Add zucchini and cook until softened, about 5 minutes. Season with herbes de Provence, salt, and pepper, stir, and cook for 1 more minute. Add drained potatoes and tomato and cook for 2 minutes.
Mix Parmesan cheese and Gruyere cheese in a bowl. Stir in half the cheese mixture into potato-vegetable mixture and transfer to the baking dish.
Melt remaining 2 tablespoons butter and mix with remaining cheese mixture and bread crumbs. Distribute evenly on top of potato-vegetable mixture in the baking dish.
Bake in the preheated oven until cheese is browned and crusty, 15 to 25 minutes.