Preheat your oven to 400°F (200°C).
In a large pan over medium heat, add olive oil and cook the shredded chicken until heated through.
Add the flour to the chicken and stir for 1-2 minutes. Gradually add chicken broth, stirring constantly until the sauce thickens.
Add thawed frozen vegetables, salt, and pepper to the mixture. Cook for another 5 minutes.
Transfer the chicken and vegetable mixture into a baking dish, spreading it out evenly.
Spread the prepared mashed potatoes over the chicken layer, smoothing it out with a spatula.
Bake in the preheated oven for about 25-30 minutes, or until the top is golden brown and the pie is heated through.
Let it cool for a few minutes before serving. Enjoy your Chicken Shepherd's Pie!