Heat butter in a medium saucepan over medium flame until melted.
2. Add flour and cook, stirring for about a minute; then, stir in milk in a steady stream to create a smooth sauce. Bring to a simmer until thickened.
3. Add the wholegrain mustard and season with salt, pepper, and cayenne pepper.
4. Add half of the cheeses, and allow to melt while stirring constantly. Turn off the heat.
5. Combine the cauliflower and shredded chicken in a baking dish that can fit in your Air Fryer basket and cover with cheese sauce. Sprinkle with remaining cheese.
6. Cook in your Air Fryer for 20-25 minutes at 350 F.