Roast the florets: preheat the oven to 425 F and line a baking sheet with foil. Place the cauliflower florets on the baking sheet and drizzle them with avocado oil. Sprinkle with salt and pepper, and bake for 20 mins or until cooked but still firm.
Make the creamy sauce: melt the butter in a large saucepan over medium heat and add the flour. Cook, stirring, for about 3 minutes.
Whisk in the milk and half and half. Cook until thickened, about 2 minutes.
Turn off the heat and stir in the nutmeg, mustard powder, ½ cup shredded cheddar, and ½ cup gruyere cheese, until melted. Season with salt and pepper.
Mix in the roasted cauliflower florets and spoon the mixture into a 9x13-inch baking dish.
Top with the remaining ½ cup cheddar and ½ cup gruyere.
Reduce the oven temperature to 350 F and bake for 30 minutes or until the cheeses are melted, bubbly, and golden brown.