Preheat your oven to 375ºF.
Bring a large pot of water to a boil and add the tablespoon of salt.
Add the cauliflower florets to the boiling water and cook for 5 minutes, until they are just starting to soften. Drain the cauliflower and set aside.
In a medium saucepan, melt 2 tablespoons of butter over medium heat.
Whisk in the flour and cook, stirring constantly for one minute.
Gradually pour in the milk, whisking continuously until the mixture boils. Continue boiling for one minute while whisking.
Remove from heat. Stir in black pepper, ½ cup shredded cheddar, and grated gruyere until melted and smooth.
Pour half the cheese sauce into an 8×11-inch baking dish.
Arrange cauliflower in an even layer over the sauce.
Pour remaining cheese sauce evenly over the cauliflower.
Combine bread crumbs with remaining ½ cup cheddar cheese and sprinkle over the top.
Melt remaining 2 tablespoons butter and drizzle over the breadcrumb topping.
Bake for 25 minutes until golden brown and bubbly.
Serve warm.