I love root vegetables for the variety of presentations that come with them. Here I've layered sweet potatoes, parsnips, and carrots. They're similar in texture, so they'll cook evenly, their colors alternate nicely, and they all have a similar level of sweetness that marries well with the onion-shallot-leek jam and the cream. —boulangere
First, start the onion jam. Slice the leeks about 1/4" thick. Fine dice the onions and shallots. Mince the garlic. Melt the butter in a large skillet over medium-high heat. Add everything you just chopped up along with a sprinkle of salt. When the vegetables have begun to wilt, add the white wine or stock. Turn heat to a simmer and cook until most of the liquid has reduced to a thick-ish syrup and all the tender vegetables have turned into a sweet, smooth jam which will top the gratin. While waiting for the transformation, go ahead and prepare your baking dish and the rest of the vegetables.
Run a pastry brush over some soft butter and brush it on the inside of an 8"x8" shallow baking dish. I use one that's about 2 inches deep. Preheat oven to 350 degrees.
Peel all the vegetables. Begin slicing them quite thin - about 1/8" I like to slice them on a very acute angle for dramatic long, thin slices. See photo #1.
Now begin layering the vegetables. Start with the sweet potatoes. Lay the slices snugly against each other. Cover them with a layer of Panko, some salt and pepper, and finally drizzle 1/2 cup of cream over. Next, do the same with the parsnip slices, again followed by Panko, salt and pepper, and 1/2 cup of cream. Do the same with the carrots, finishing with one last layer of Panko and the rest of the cream. Gently spoon your onion jam over the top and smooth it to cover.
Set the baking dish on a baking sheet. Loosely cover the top of the dish with foil. Set everything in the oven. Bake for 45 minutes. Remove dish from oven, remove the foil, and scatter your grated cheese over the top (I used smoked Gouda). Return dish to oven and bake for 15 minutes more to melt and lightly brown the cheese. Gratin is done when you can pierce all the way to the bottom easily with a paring knife.
Remove the gratin from the oven and let sit for 10 minutes before serving.