Heat a large skillet over medium-high heat. Add the ground turkey and chopped onion, breaking the meat apart with a spoon. Cook until the turkey is fully browned and the onions are soft, then drain any excess liquid.

Add the minced garlic, salt, pepper, paprika, and thyme to the skillet. Cook for 1 minute until fragrant.

Reduce the heat to low. Stir the cream of mushroom soup and milk directly into the skillet with the turkey. Mix well until it forms a creamy meat gravy, then remove from heat.

Arrange half of the sliced potatoes in an even layer at the bottom of the prepared baking dish. Pour half of the creamy turkey mixture over the potatoes, spreading it out to the edges. Sprinkle the frozen peas evenly over the meat, followed by 1 cup of the cheddar cheese.

Arrange the remaining potatoes in a layer on top, followed by the remaining turkey mixture.

Cover the dish tightly with foil—this is important to trap the steam so the potatoes cook through. Bake for 45–50 minutes. Remove the foil. The potatoes should be fork-tender. Sprinkle the remaining 1 cup of cheese over the top and bake uncovered for another 10–15 minutes until the cheese is melted, bubbly, and lightly golden.

Let the casserole rest for 10 minutes before serving to let the sauce set up.