Bring a large pot filled with water to a boil over high heat; salt it generously. Add the macaroni and cook according to package instructions. Drain and set aside.
Preheat the broiler to high.
Heat a Dutch oven or heavy-bottomed saucepan over medium heat. Melt 2 tablespoons of the butter, then add the onions. Cook until the onions soften, 2 to 3 minutes. Then stir in the garlic, mustard, 1/2 teaspoon salt and the pepper and cook for another minute. Add the remaining 1 tablespoon butter to the pan to melt. Sprinkle the flour over the butter and stir until it turns a deep blonde color and has a nutty aroma, about 1 minute. Slowly whisk in the milk. Cook, whisking frequently, until it starts to thicken. Then add the cream and thicken for another 2 to 3 minutes. Stir in the cheeses until melted. Add the crawfish to the sauce, and then fold in the macaroni until coated. Remove from the heat.
Divide the mac and cheese among 8 large ramekins or cast-iron skillets. Broil until the tops of the noodles are golden brown and bubbly. Let cool slightly, then serve warm.