Preheat your oven to 375°F (190°C).
Prepare the topping. In a small saucepan or microwave-safe bowl, melt 4 tablespoons of butter with 1 minced garlic clove. Place the bread cubes in a large bowl, drizzle with the garlic butter, and toss to coat. Set aside.
In a large, 12-inch oven-safe skillet, heat the olive oil over medium-high heat. Add the mushrooms and cook, stirring occasionally, until browned and their liquid has evaporated, about 5-7 minutes. Transfer the mushrooms to a plate.
Reduce the heat to medium and melt the remaining 4 tablespoons of butter in the same skillet. Add the onion and carrot and cook until softened, about 5 minutes. Stir in the remaining 2 minced garlic cloves and cook for one more minute until fragrant.
Sprinkle the flour over the vegetables and stir constantly for 1 minute to form a paste.
Slowly pour in the milk while whisking continuously to prevent lumps. Add the bouillon powder, salt, and pepper. Bring the sauce to a simmer and cook, stirring frequently, until it has thickened enough to coat the back of a spoon, about 3-4 minutes.
Remove the skillet from the heat. Stir in the chopped turkey, frozen peas, and the cooked mushrooms until everything is evenly combined in the sauce.
Arrange the prepared bread cubes evenly over the top of the filling. Sprinkle with Parmesan cheese, if using, and then the shredded cheddar cheese.
Bake for 20-25 minutes, until the filling is hot and bubbling and the cheese is melted and golden brown. For poultry, a safe internal temperature is 165°F (74°C). Let the pot pie rest for 5 minutes before serving.