In a medium sauce pan, melt the butter over medium heat. Whisk in the flour and cook until bubbly. Slowly whisk in the reserved chicken stock, stirring vigorously until smooth. Stir in the milk, thyme, sage, salt and pepper. Bring sauce to a boil; reduce heat and simmer until thickened and bubbly, about 10 minutes. Stir in the parmesan and sour cream. Set sauce aside.
Heat the olive oil over medium heat. Add the chopped celery, carrots and onion, and saute until onion is translucent and veggies are softened, about 5 minutes. Remove from heat. Stir in the chopped chicken and frozen peas. Pour the prepared sauce over the chicken mixture, stirring well to combine. Pour mixture into a greased 9x13" baking dish.
Unroll the crescent roll dough, pinching the seams together to seal. Use a knife to cut the dough into 1" strips. Arrange the dough strips in a lattice pattern on top of the filling. Bake at 375 for 25-30 minutes, until filling is bubbly and crust is golden.