Break the cauliflower down into bite size florets. In a large pot of salted boiling water, blanche the cauliflower for approx. 5 minutes until tender but still firm. Drain.
In a saucepan, melt 2 Tbsp of the butter. Add the flour and make a rue, stirring constantly for approx. 2 minutes. Once the flour is cooked, add the milk to the rue mixture. Boil, whisking constantly for about 1 minute until thickened. Remove from heat.
Add the salt, pepper and half of the gruyère, and the parmesan. Stir until all the cheese is melted.
In a 8 x 11 baking dish, arrange the drained cauliflower evenly in the dish. Pour the sauce over the cauliflower.
Combine the remaining 1/4 cup of the gruyère cheese with the breadcrumbs and sprinkle over top of the cauliflower.
Melt and drizzle the remaining butter over the gratin.
Bake for 25 to 30 minutes in a 375 degree oven.