Preheat the oven's broiler and set the oven rack about 6 inches from the heat source. Grease a 2-quart casserole dish with cooking spray.

Melt 2 tablespoons of butter in a saucepan over medium heat. Stir in the chopped onion and shallots; cook and stir until the onion has softened and turned translucent, about 5 minutes. Pour in the chicken stock and bring to a boil over high heat. Reduce heat to medium and simmer until the chicken stock has reduced to half its original volume, about 5 minutes. Pour in the heavy cream; whisk in the Dijon mustard, horseradish, cayenne pepper, paprika, salt, and pepper. Bring to a simmer and cook 3 to 5 minutes, stirring occasionally.

While the sauce is cooking, melt 3 tablespoons of butter in a skillet over medium heat. Stir in the garlic and green onions; cook and stir until the aroma of the garlic has mellowed, about 5 minutes. Add the crawfish tails and cook until hot. Stir the crawfish mixture into the sauce along with the Parmesan cheese. Pour into the prepared casserole dish and sprinkle with bread crumbs.

Cook under the preheated broiler until the bread crumbs are golden brown. Let stand for a few minutes before serving.