Preheat your oven to 425°F (220°C).

In an 8"x8" baking dish, place the cauliflower florets. Drizzle with olive oil and season with salt, pepper, garlic powder, and dried rosemary. Toss to coat and arrange in a single layer. Roast for about 10 minutes.

In a saucepan over medium heat, melt butter. Whisk in flour and cook for about 1 minute. Gradually pour in cream or milk, stirring continuously for 4–5 minutes until thickened.

Remove from heat and stir in mozzarella, cheddar, garlic powder, pepper, and mustard. Mix until melted and creamy.

Pour the cheese sauce over the roasted cauliflower, ensuring even coverage.

Reduce oven temperature to 350°F (175°C) and bake for about 20 minutes until bubbly and fully tender.

Sprinkle additional shredded cheese on top and broil for 2–3 minutes until golden brown.

Sprinkle with chopped chives and let cool for a few minutes before serving.