1 Cook the leeks: Melt butter on medium heat in a medium sized saucepan. Add the sliced leeks and gently cook until completely softened, about 8 to 10 minutes.
2 Lightly steam the cauliflower: While the leeks are cooking, place cauliflower florets in a steamer basket in a saucepan over an inch of water. Bring water to a boil, cover, and steam the cauliflower for 3 to 4 minutes.
The steaming should just take the rawness out of the cauliflower, but not cook it so much that it is tender. You want the cauliflower at this stage to be al dente. Remove strainer and cauliflower from hot pan and set aside.
3 Make roux with butter, leeks, flour, add milk: Add 2 Tbsp of flour to the leeks and butter. Stir and let cook for a minute or two. Slowly add the milk, stirring as you add it to break up any lumps. Bring to a low simmer and continue to stir as the sauce thickens.
4 Stir in cheese, thyme, nutmeg, salt: Once the sauce is thick enough to coat the back of a spoon, stir in 3 ounces of the shredded cheese (reserving the rest of the cheese for the topping), the thyme, nutmeg, and salt.
5 Preheat oven to 400°F.
6 Pour sauce over cauliflower in gratin dish: Spread a teaspoon of butter over the insides of a 2-quart gratin dish. Place the lightly steamed cauliflower florets in an even layer in the dish. Pour the leek and cheese sauce over the cauliflower. Top with remaining cheese.
7 Bake at 400°F for 25 minutes.
8 Toast breadcrumbs: While the gratin is baking, melt 1 Tbsp of butter in a small sauté pan on medium high heat. Add the fresh breadcrumbs and toast in the melted butter for 4 to 5 minutes, until lightly browned. Sprinkle with 1/4 teaspoon of salt, and a sprinkling of pepper. Remove from heat.
9 Top gratin with toasted breadcrumbs and bake: After the gratin has baked for 25 minutes, remove it from the oven. Sprinkle toasted breadcrumbs over the top and return to the oven. Bake for 5 more minutes.