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Preheat oven to 350 degrees F.
In a large saucepan over low heat, melt the butter. Slowly whisk in the flour and cook for 1 to 2 minutes, until the mixture is creamy and thick. Slowly stir in the milk and mix until smooth, about 1 minute. Add the 1/2 cup of grated cheese and stir until melted and well-mixed, about 1 minute. Add the onion, garlic, salt, celery, mustard and sugar to the sauce and mix well. Remove the sauce from heat and pour over the christophene in the baking dish. Stir everything together, then top with the remainig 1 cup grated cheese.
Place the mixture in the oven and bake until golden brown, about 20 to 30 minutes. Serve warm.