Preheat oven to 400° F.  Butter a 7 by 7-inch baking dish; set aside.

In a medium saucepan, melt 2 Tablespoons butter over medium heat. Add the flour and cook, stirring constantly, until thickened into a pale blond roux, about 2 minutes. Add the hot chicken broth in a steady stream, whisking to incorporate. Bring to a simmer and cook, stirring frequently with a whisk or heavy wooden spoon until thick. Add salt and pepper, and stir to incorporate. Slowly whisk in the milk and simmer, stirring, until well incorporated and thickened. Remove from the heat and very lightly film the top with the milk to prevent a skin from forming.

In a large sauté pan or skillet, heat the remaining 2 Tablespoons of butter over medium-high heat. Add the diced chicken, season lightly with Essence, and cook, stirring, until lightly browned and cooked through, about 4 minutes. Remove with a slotted spoon. To the fat and juices in the pan add the shallots and garlic and cook, stirring, for 1 minute. Add the mushrooms, and cook, stirring, until they have given off their water and are starting to color, 3 to 4 minutes. Add the ham and cook, stirring, for 1 minute. Add the sherry and bring to a boil, stirring to deglaze the pan. Add the tarragon and return the chicken to the pan. Stir well to blend and cook until the sherry has evaporated. Reduce the heat to low and stir in enough of the white sauce to bind and thicken, about 1 cup. Remove from the heat and let cool slightly.

Place the crêpes on a work surface. One at a time, spoon the filling along the bottom third of each crêpe, about 3 tablespoons per crêpe, and roll into a cylinder to enclose the filling. Place, seam side down, in the prepared baking dish. Repeat with the remaining crêpes.

To the remaining white sauce, add the Gruyère and stir to incorporate. Spoon the sauce evenly over the crêpes. (NOTE: The crêpes can be tightly covered with plastic wrap and refrigerated for 1 day or frozen for 3 days at this point. Bring to room temperature before baking.)

Top with grated Parmesan cheese, and bake at 400° F until warmed through and the top starts to become bubbly, 20 to 25 minutes. Remove from the oven and top with the fresh chives. Bake until the cheese is melted and the topping is golden brown, 5 to 10 minutes.

Remove from the oven and place in a chafing dish holder or on trivets. Serve hot, 1 to 2 crepes per person. Yield: 4 servings.

Crêpes:
1 large egg
⅛ teaspoon salt
1 Tablespoon cooking oil
½ teaspoon sugar
6 Tablespoons all purpose flour
½ cup milk

Beat egg, salt, oil and sugar in a mixing bowl with whisk until blended. Add flour and milk alternately, beating until smooth. When ready to make crepes, spray a 7-inch frying pan with non-stick spray. Heat over medium high heat until hot. Pour 3 Tablespoons batter into pan; quickly tilt to make a thin layer. Brown lightly on both sides. Let cool on paper towel.  Yield: 5 to 6 crêpes.