Begin by preheating your oven to 375°F (190°C). To prepare the filling, melt 4 tablespoons of butter in a heavy pan over medium heat, then add the 12 ounces of quartered cremini mushrooms.
Cook while stirring often for 2–3 minutes until the mushrooms begin to soften and release their moisture. Sprinkle ⅓ cup of all-purpose flour over the mushrooms and stir constantly for 1 minute to create a roux base.
Remove the pan from heat and gradually blend in 1¼ cups of chicken stock, stirring smoothly to avoid lumps.
Return the pan to the heat, bring the mixture to a boil while stirring, and simmer for 2–3 minutes until the sauce thickens.
Fold in the 12 ounces of bite-sized cooked chicken pieces and 1 tablespoon of fresh chopped tarragon, then season to taste with salt and black pepper.
While the filling simmers, prepare the spinach component. Rinse the 1 pound of coarsely chopped spinach leaves and place them in a saucepan with only the water clinging to the leaves.
Cook for 2 minutes or until the spinach is tender, then drain thoroughly, squeezing out any excess moisture with the back of a spoon. Stir in 2 tablespoons of butter and a pinch of grated nutmeg to the warm spinach, blending until the butter is fully incorporated and the nutmeg is evenly distributed.
To assemble, lay out the 8 crêpes and divide the spinach mixture evenly among them, spooning it onto one half of each crêpe.
Top the spinach with the chicken and mushroom filling, then fold each crêpe in half. Arrange the filled crêpes in a buttered baking dish, sprinkle the 4 ounces of grated Gruyère cheese over the top, and bake for approximately 15–20 minutes until the cheese is melted and golden and the crêpes are heated through.
