Heat the vegetable oil in a large skillet over medium heat.
Season the mahi mahi fillets with the thyme, salt, and black pepper; lay the seasoned fillets in the hot skillet. Cover the skillet and cook the fillets for 7 minutes, turn the fillets, and continue cooking the other side until the flesh flakes easily with a fork, 5 to 7 minutes.
Remove the mahi mahi to a cutting board and cut into bite-size pieces, removing any visible bones as you notice them.
Stir the cream of potato soup, milk, and mixed vegetables together in a saucepan over medium heat; season with salt and pepper. Allow the mixture to simmer, stirring occasionally, 5 minutes.
Gently fold the mahi mahi pieces into the soup mixture.
Line a 9-inch deep-dish pie plate with one of the prepared pie crusts.
Pour the soup mixture with the mahi mahi into the pie crust.
Lay the remaining pie crust atop the assembled pie. Seal the edges with wet fingertips.
Bake in the preheated oven until the crust is a golden brown and the pie is heated through, 40 to 45 minutes. Allow to cool 10 to 15 minutes before serving.