Preheat oven to 350 degrees F.

Stuffing: Saute chanterelles, leeks, and onions until transparent. Add wine and let boil and reduce until almost dry. Add heavy cream and reduce until thick. Season with salt and pepper. Let cool completely.

Make a slit on side of breast and using the back of a wooden spoon, open the slit into a pocket. Stuff it with the mushroom mixture.

In a hot, ovenproof skillet, saute chicken in butter. Turn when slightly brown and place in oven and bake for 5 minutes. Remove from oven, pour off excess grease, and set breasts aside to rest.

Place pan on stove, add the leeks, onion, and fennel. Let cook until translucent. Add the garlic and Herbes de Provence and stir until aromatic. Deglaze with the Chablis and season with salt and pepper. Let simmer for 5 minutes. Remove from heat and let cool.

Roll out the puff pastry dough and halve width-wise into two short rectangles. Place a chicken breast in the bottom half of the dough. Top with half the fennel sauce. Fold the long part of the dough over the chicken and press the edges to seal with a fork. Brush with egg white wash. Poke a design in the top of the crust with a knife. Bake for 15 minutes.