Pre-heat oven to 400 and lightly grease 2 quart square baking dish. Sit aside.
In medium sized mixing bowl add small bite size pieces of chicken breasts, 1/2 teaspoon garlic powder, 1/2 teaspoon onion powder, rosemary, Paul Prudhomme Poultry Magic seasoning, 1/4 teaspoon sea salt and 1/4 teaspoon fresh ground black pepper and extra virgin olive oil. Stir until all pieces are covered in oil and seasonings. Heat remaining oil in skillet on medium - medium high heat. Add chicken and brown until light golden brown about 5 minutes (remember chicken will also be baked for at least 25 minutes). Cover a plate with a paper towel and using a slotted spoon put chicken on paper towel to drain oil. Make sure to cover with another paper towel so chicken won't dry out.
On a lightly floured surface roll out the first sheet of puff pastry to 12 inch square. If the pastry cracks use a dab of water on your finger tip and lightly push to reseal. Take the pastry and lay in the casserole dish. Position until all sides are even. Be careful not to tear but you don't need to handle with kid gloves either. It's always easy to reseal any tear. Trim any excess that hangs over the edges of the casserole dish. Cover with foil and bake for 25 minutes or until all darker, doughy areas are lighter and puffed. While the puff pastry is baking you can multi-task by making the vegetables and sauce.
In a skillet melt butter on medium-medium high heat. Add vegetables and cook until tender crisp. About 5-7 minutes. Don't over cook because vegetables will be cooking while you make the sauce and also baking in the oven later. You don't want them to turn to mush. Add flour and stir cooking until flour turns golden brown approximately 3 minutes. Slowly add chicken broth, ground dry mustard, remaining salt, remaining fresh ground pepper, remaining garlic powder, remaining onion powder, 1 teaspoon parsley flakes, peas and corn starch (no need to make a slurry). Reduce to simmer and stir constantly until sauce thickens. If it's too thick add a little extra chicken broth. Sit aside and wait until bottom shell is finished baking.
Re-flour your surface and place 2nd puff pastry sheet. Using a pizza cutter cut into 1/4 inch strips.
Once bottom shell is finished baking add chicken and sauce mixture. Place puff pastry strips on top using a weave design (or whatever design you feel like making. I haven't mastered the weave yet but I'm working on it). Trim any excess puff pastry. Sprinkle with remaining parsley flakes. Place casserole on foil lined baking sheet and place in oven. Cook for 25 minutes or until sauce is bubbly and puff pastry is golden brown.