Cheesy Winter Vegetable Gratin

Preheat the oven to 375 degrees F (190 degrees C).

Cut leeks in half lengthwise and wash well. Cut white and light green parts into 3-inch lengths, keeping the layers together. Discard dark green tops. Arrange leeks in a 9x13-inch glass baking dish along with potatoes, carrots, cauliflower, Brussels sprouts, thyme, parsley, and bay leaf.

Melt butter in a skillet over medium-high heat. Saute onion until golden, about 8 minutes. Stir in water, salt, and pepper. Bring to a boil and pour over the vegetables. Cover the dish lightly with foil.

Bake in the preheated oven until vegetables are tender, 45 to 55 minutes.

Remove foil and sprinkle Swiss cheese evenly over the vegetables. Continue baking until cheese is melted, bubbly, and starting to brown, about 15 minutes more. Discard herbs before serving.