Fill a large pot with water, place over medium-high heat and bring to a boil. Once water is boiling add cauliflower. Cook cauliflower for 5 to 6 minutes or until tender but there’s still a bite.
Remove cauliflower from heat and drain into a colander. Transfer to a large bowl.
Place a saucepan over medium heat and melt butter.
Add flour to butter and stir together until no lumps remain, 2 to 3 minutes.
Stir milk and Dijon into mixture and stir until mixture is smooth and thick enough to coat the back of a spoon, about 3 minutes.
Remove saucepan from heat and stir in cheese, a small handful at a time. Allow cheese to completely melt before adding in more cheese. Continue to add cheese and stir until all cheese has been added and sauce is smooth and creamy. Season with salt and pepper to taste.
Pour cheese sauce over cauliflower and toss together until evenly coated. Finish with black pepper and serve.
