Basic Vegetable Gratin
Source: Recipes from America's Small Farms: Fresh Ideas for the Season's Bounty by Joanne Hayes, Lori Stein and Maura Webber

The only thing that makes a casserole a gratin is the crisp, well-browned, broiled topping. You can use buttered bread crumbs, grated cheese, a mixture of the two, or nothing at all over layers of cooked vegetables.

1 pound potatoes, turnips, rutabagas, sweet potatoes, or Jerusalem artichokes, peeled and thinly sliced
1 pound leafy greens, cabbage, zucchini, summer squash, fennel, Belgian endive, or cauliflower, rinsed and drained, if necessary, and thinly sliced, or an additional pound of roots and tubers above
4 tablespoons olive oil or unsalted butter
1 large onion, chopped
2 garlic cloves, finely chopped (optional)
2 cups milk, stock, or cooled vegetable cooking liquid
1/4 cup all-purpose flour
Salt and freshly milled black pepper
1 1/4 cups grated Cheddar, Swiss, Muenster, Monterey Jack, or other cheese
1/2 cup fresh bread crumbs or panko (Japanese bread crumbs)

Cook the root vegetables in boiling salted water for 5 to 7 minutes, until the surface starts to look cooked. Drain; save the cooking liquid and let cool to use for the sauce, if desired. Blanch the pound of more tender vegetables; drain thoroughly.

Preheat the oven to 375F. Lightly grease a 2-quart gratin or shallow baking dish.

Heat 2 tablespoons of the oil in a skillet over medium heat. Saute the onion and garlic, if using, until they start to brown, about 4 minutes.

Whisk the milk into the flour in a small bowl. Whisk the milk mixture into the onion mixture and cook, stirring constantly, until the sauce is bubbly and thickened. Add salt and pepper to taste.

Layer half of the root vegetables, 1/3 cup sauce, 1/4 cup cheese, half of
the tender vegetables, 1/3 cup sauce, and 1/4 cup cheese. Repeat, ending with
1 cup sauce and 1/2 cup cheese.

Combine the bread crumbs and the remaining 2 tablespoons oil. If using butter, melt the remaining 2 tablespoons before combining. Sprinkle crumbs over the cheese.

Bake for 35 to 40 minutes, until the root vegetables are tender and the top is well browned.