Preheat the oven to 400F.
Add your cauliflower florets to a large mixing bowl with some Kosher salt, olive oil, and freshly-ground black pepper.
Grab two large sheet trays and line them with parchment paper.
Spread the seasoned and oiled cauliflower florets onto the sheet trays, leaving some space between the florets so that they roast evenly, without steaming.
Evenly roast your seasoned cauliflower for about 20 minutes, stirring the cauliflower halfway through to encourage more even browning.
Keep a close eye on the cauliflower florets as they roast and take on a beautiful golden-brown color.
Make sure the florets are golden-brown and tender.
Once done, remove the sheet trays with the cauliflower from the oven, and set aside to let the cauliflower florets rest at room temperature.
Reduce the oven temperature to 350F.
Heat up a blend of heavy cream and whole milk in a small sauce pot on the stove until it is warm.
Remove the milk and heavy cream from the stove, and keep warm.
Melt the butter over medium heat in another small sauce pot, allowing it to warm up, before adding the all-purpose flour.
Stir this mixture with a rubber spatula and continue to cook for about 7-10 minutes, or until the roux is cooked and smells like lightly-toasted nuts and browned butter.
Whisk in the warm whole milk and heavy cream mixture slowly into the butter and flour roux mixture.
*Add a little bit at first, and continue to add in small amounts until the milk and heavy cream mixture is fully incorporated into the roux, with no visible lumps.
As the mixture thickens, continue stirring and cooking for an additional ten minutes over low heat.
Turn the heat to the lowest setting, and add the shredded cheeses into the bechamel mixture of butter, flour, milk, and heavy cream.
*As the cheese melts, the sauce will transform into a thick and velvety sauce.
Taste the sauce mixture, and season with Kosher salt & freshly-ground black pepper.
Add the roasted cauliflower from earlier to a large mixing bowl.
Pour the sauce into a large bowl filled with the roasted cauliflower.
Toss gently to coat the cauliflower in the bowl with the cheese sauce, and transfer the mixture back to a sheet tray lined with parchment paper.
Bake at 350F for an additional 5 minutes, or until the cheese sauce is hot and intermingled with the warmed roasted cauliflower.
Remove the baked cauliflower and cheese sauce from the oven, and serve onto plates.
Garnish with fresh parsley, and serve hot.