Place the chicken breasts on a baking sheet and rub them with olive oil. Sprinkle generously with salt and pepper. Roast for 30 minutes, or until cooked through. Set aside until cool enough to handle, then cut the chicken into large dice.
While chicken is baking, boil water in small pot and cook potatoes for about 15-20 minutes. Once potatoes are fork tender, drain in a colander.
In a small saucepan, heat the chicken stock and dissolve the bouillon cubes in the stock.
In a large pot, melt the butter and saute the mushrooms and onions over medium-low heat for 10 to 15 minutes, until translucent. Add the flour and cook over low heat, stirring constantly, for 2 minutes. Add the hot chicken stock to the sauce. Simmer over low heat for 1 more minute, stirring, until thick. Add 2 teaspoons salt and 1/2 teaspoon pepper. Add the cubed chicken, carrots, peas, potatoes, mushrooms, onions and broccoli. Mix well.
Lightly spray 9-inch pie dish with non-stick cooking spray. Carefully unroll one pie crust and press down and onto sides of pie dish. Using a pastry brush, spread butter on the crust. Pour chicken mixture onto crust until it almost reaches the top of the dish. Carefully unroll the second pie crust and place on top of the chicken mixture. Pinch edges together. Brush the top of the pie crust with butter. Using a knife, make a slit about 1 to 2 inches on top of pie crust. Bake for 1 hour or until crust is light golden brown. Brush more butter on top of crust, if desired.