Set a large pot over water over high heat and bring to a boil. Add the cauliflower florets and cook for 5-6 minutes until tender, then drain and let it slightly cool to release the steam.
While the cauliflower cooks, combine the chicken broth and cream in a bowl and warm in the microwave for 1 minute. Melt 2 tablespoons of butter in a medium saucepan over medium heat. Add flour and cook, stirring constantly, for 1-2 minutes until lightly browned. Add the warm cream and broth and whisk for around 1-2 minutes until thickened and starting to bubble.
Remove from heat and season with salt, pepper, and nutmeg. Then add in ½ cup grated Gruyere cheese and ½ cup grated Parmesan cheese and stir until smooth.
Coat the bottom of your baking dish with about a third of the sauce. Add the cauliflower, then pour the remaining sauce evenly over top. In a small bowl, melt the remaining butter and combine with the Panko breadcrumbs and the remaining ½ cup of Gruyere cheese. Sprinkle the topping evenly on top.
Bake for around 25 minutes until browned and bubbling then remove and serve.
Notes
Store leftovers in an airtight container and refrigerate for up to 5 days.Reheat in a 325°F oven until warmed through. Smaller portions can be reheated in the microwave.