Preheat your oven to 375ºF.
Bring a large pot of water to a boil and add the tablespoon of salt.
Add the cauliflower to the pot and boil for 5 minutes. The florets will be slightly soft. Drain in a colander and set aside.
Melt two tablespoons of the butter in a medium sized pot over medium heat.
Add the flour to the pot and whisk, stirring constantly for one minute. The flour and butter mix may start to turn brown but this is okay—it’s flavor!
Pour the milk into the pot and whisk until the milk begins to boil. Boil for one minute, whisking constantly.
Remove the pot from the stovetop and add the pepper, ½ cup cheddar cheese and gruyere. Whisk until the cheese melts.
Pour half of the cheese sauce in the bottom of a 8x11 baking dish and spread it across the whole dish.
Add the cooked cauliflower florets to the pan, making an even layer across the bottom.
Pour the rest of the cheese sauce over the cauliflower florets.
In a small bowl, toss together the breadcrumbs and remaining ½ cup of cheddar cheese. Sprinkle this mix over the cauliflower in the pan.
Melt the remaining 2 tablespoons of butter and drizzle it over the top of the breadcrumbs.
Bake the cauliflower gratin for 25 minutes in the oven. The top should be completely golden brown and the cheese sauce will be bubbling in the pan.
Serve while hot!