Preheat your oven to 190 C. Line a baking sheet with foil.
Place the chicken breasts on the baking sheet and rub them with olive oil on both sides, then position them skin side up. Sprinkle generously with salt and pepper. Roast for 35 to 40 minutes, or until cooked through. Set aside until cool enough to handle, then remove the skin. Cut the chicken into large cubes. You will have 4 to 6 cups of cubed chicken.
In a large pot melt the butter and saute the 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 chicken stock. Simmer over low heat for 1 more minute, stirring, until thickened. Add 2 teaspoons salt, 1/2 teaspoon pepper, and the milk.
Then add the cubed chicken, carrots, peas, and parsley. Mix well. Place the stew large, deep casserole baking dish. Place the baking dish on a sheet pan and bake for 15 minutes.
Meanwhile, make the biscuits. Combine the flour, baking powder, salt, and sugar in the bowl of an electric mixer fitted with the paddle attachment. Add the butter and mix on low speed until the butter is the size of peas. Add the milk and combine on low speed. Mix in the parsley. Dump the dough out on a well-floured board and, with a rolling pin, roll out to 3/8-inch thick. Cut out twelve circles with a 2 1/2 inch round cutter.
Remove the stew from the oven and arrange the biscuits on top of the filling. Brush them with egg wash mix 1 egg with a tablespoon of water), and return the dish to the oven. Bake for another 20 to 30 minutes, until the biscuits are brown and the stew is bubbly.
Remove from oven, let rest 5-10 minutes, serve.