Preheat oven to 375 degrees.
Cook the cauliflower florets in a large pot of boiling water for 5-6 minutes until tender but still firm. Drain and set aside.
Melt 2 tablespoons of butter in a medium saucepan over low heat. Add the flour, stirring constantly for 2 minutes.
Add hot milk into the butter-flour mixture and whisk constantly, for one minute, or until thick.
Add 1 teaspoon of salt, pepper, and 1/2 cup of the Gruyere to the mixture.
Pour ⅓ of the sauce on the bottom of an 8 x 12 casserole dish. Place the drained cauliflower on top and spread remaining sauce on top.
For the topping, combine bread crumbs with the remaining 1/4 cup of Gruyere and sprinkle on top. Melt the remaining butter and drizzle over top.
Sprinkle with salt and pepper and bake for 30 minutes or until top is browned.