I'm just copy-pasting the recipe over here from The Wednesday Chef, but do go over there to read her fabulous description of it because it'll totally make you hungry.
Note: I used fontina instead of Gruyere, and it filled a 9x13 glass baking dish perfectly.
Cauliflower and Potato Gratin
Serves 6 to 8 as a side dish
1 (1 1/4-pound) head of cauliflower
1/2 pound small boiling potatoes
1 tablespoon vinegar
1/4 cup butter
1 leek, white part only, finely chopped (about 1/2 cup)
1/4 cup flour
1 1/2 cups milk
2 tablespoons crème fraiche or sour cream
3/4 cup grated Gruyère cheese, divided (about 1 1/2 ounces)
Freshly grated nutmeg
1/4 cup fresh bread crumbs
1. Heat the oven to 400 degrees. Cut the dried base and green leaves from the cauliflower and discard them. Separate the head into florets about the size of walnuts and chop the stem into similar size pieces. Cut the potatoes into similar-size pieces as well.
2. Bring a large pot of water to a boil over high head and salt liberally. Add 1 tablespoon of vinegar and the cauliflower and potatoes (the vinegar will help keep the cauliflower white). Cook until the cauliflower pieces are tender enough to be easily cut with a spoon, 10 to 15 minutes. Drain and set aside.
3. While the cauliflower is cooking, make a cheese sauce. In a medium, heavy-bottom saucepan, melt the butter over medium-low heat and stir in the leeks. Cook until they are soft, about 10 minutes. Add the flour and whisk to make a smooth paste. Add the milk a little at a time, cooking until it thickens. When all the milk has been added, reduce the heat and cook over medium-low heat for 5 to 10 minutes.
4. Stir in the crème fraîche, then one-half cup of the Gruyère, 1 teaspoon salt and a generous grating of nutmeg (a little less than one-quarter teaspoon). Whisk until smooth, then taste and add more salt or nutmeg if necessary.
5. Butter a 6-cup gratin dish and spread a thin layer of the sauce evenly over the bottom. Arrange the cooked cauliflower and potatoes in an even layer over the sauce. Pour the remaining sauce over the top and spread evenly with the back of a spoon. It should come about three-quarters of the way up the vegetables.
6. Scatter bread crumbs evenly over top and then scatter the remaining one-quarter cup Gruyère over that. Bake until the gratin is bubbling and the top is browned, 30 to 40 minutes. Serve immediately.