Preheat grill to 375 degrees Fahrenheit using two-zone or indirect grilling.
Cook pasta according to package instructions until al dente. Set aside.
In a 10-inch cast iron skillet, melt butter and flour over medium-low heat to form a roux or thick paste. Stir for 3 minutes, until a thick paste, but not browned.
Add milk to the roux, whisking continuously until the mixture comes to a boil. Then reduce heat to a simmer, allowing the sauce to thicken (about 5 minutes). Add the crème fraiche, Dijon mustard, nutmeg, salt and pepper. Whisk together to combine over low heat.
Turn off heat. Mix in the cheddar and fontina cheeses to the sauce, and combine. The cheese will melt quickly. Add half the bacon (reserving the other half for panko topping), macaroni, and all the chicken. Mix well to combine.
Top the mac and cheese with the panko parmesan bacon topping.
Place the skillet in the grill over indirect heat, and cook for 25 minutes uncovered until the the sides are bubbling and the top is golden and crispy. Remove and serve warm.