Cook pasta shells to al dente according to package instructions. Cook a few extra in case some tear badly. Handle gently to avoid damage to the shells. When done cooking rinse gently in cold water.

Preheat oven to 350 degrees. Spray a 9 X 13" baking pan with cooking spray.

Pulse the chicken, walnuts and vermouth in food processor until they are uniformly small pieces. (Not a paste but a fine texture).

With a tablespoon fill each shell with chicken mixture and position filled shells in baking pan.

If you have chicken mixture left over sprinkle it evenly over the top of the shells in the baking pan.

Melt the butter and saute the mushrooms about 8 minutes until the juices are released and slightly evaporated.

Remove the mushrooms and set aside. Add the chicken broth, vermouth if using and tarragon sprigs to the pan and simmer about 8 minutes.

Return mushrooms to the pan and and the cream. Heat through. Remove the tarragon sprigs. Add the fontina and gran padano cheeses stirring to melt evenly.

Make a slurry by adding the 4 Tbsps cream to the 4 Tbsps flour. Mix into the sauce stirring constantly. Simmer til thickened.

Pour the sauce over the stuffed shells and bake about 30 minutes till heated through.

Serve hot.