Set your oven to 375°F before you start cooking. This gives it time to heat fully while you work. A hot oven helps the potato top turn golden.
Chop the onion, pepper, carrot, and celery into small, even pieces. Mince the garlic and chop the turkey into bite-size chunks. Having everything ready makes the cooking part smooth and fast.
Melt the butter in a large skillet over medium heat. Add the onion, bell pepper, carrot, and celery. Cook for about 5 minutes, stirring now and then, until they turn soft.
Stir in the basil, thyme, salt, pepper, and minced garlic. Cook for about 30 seconds, just until you smell the garlic. Don't let it brown, or it will turn bitter.
Sprinkle the flour over the veggies and stir for 1 to 2 minutes. Pour in the broth and add the bouillon cube, stirring until it dissolves. Let it simmer for about 5 minutes until the sauce thickens.
Add the chopped turkey and frozen peas to the skillet. Mix well so everything is coated in the gravy. Bring the filling to a gentle boil, then turn off the heat.
Spoon the mashed potatoes over the filling and spread them to the edges. Smooth the top with the back of your spoon. Drag a fork across the surface to make ridges, then sprinkle on the mozzarella.
Move the skillet to the oven and bake for 30 minutes. You want bubbling edges and a golden, melted cheese top. Keep an eye on it near the end so it doesn't over-brown.
Let the pie rest for 5 minutes after it comes out. This helps the filling set so it scoops neatly. Sprinkle with parsley and serve warm.