Step 1Heat 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.
Step 2Bring 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.
Step 3While 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.
Step 4Stir in the creme fraiche and then one-half cup of the Gruyere, 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.
Step 5Butter 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.
Step 6Scatter bread crumbs evenly over top and then scatter the remaining one-quarter cup Gruyere over that. Bake until the gratin is bubbling and the top is browned, 30 to 40 minutes. Serve immediately.