- 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.