Preheat the oven to 400°F. Thaw the frozen sweet potatoes, corn, and peas. Dice the onion, carrots, and celery. Mince the garlic. Finely chop the rosemary and remove the thyme leaves from the stems.

In a large skillet or Dutch oven, heat the olive oil and butter over medium heat. Add the onion, carrots, celery, and kosher salt. Cook for 8 to 10 minutes, stirring occasionally, until the vegetables begin to soften and the onion is translucent.

Add the garlic, half of the thyme, and half of the rosemary. Cook for 1 to 2 minutes, stirring often, until fragrant.

Sprinkle the flour over the vegetables. Stir until the vegetables are evenly coated. Cook for 2 to 3 minutes, stirring frequently, until the flour absorbs the fat and smells lightly toasted.

Slowly pour in the chicken stock while stirring to prevent lumps. Add the milk and stir again. Bring the mixture to a gentle simmer and cook for 4 to 6 minutes, stirring often, until thick enough to coat the back of a spoon.

Stir in the thawed sweet potatoes, corn, and peas. Cook for 2 to 3 minutes, just until warmed through. Stir in the remaining thyme and rosemary. Taste and add more salt if needed.

Pour the filling into a pie dish or baking dish. Lay the pie crust over the top. Trim or tuck the edges as needed. Cut a few small slits in the center to let steam escape.

Brush the crust with milk or melted butter if desired. Sprinkle with extra chopped rosemary.

Bake for 25 to 35 minutes, until the crust is golden brown and the filling is bubbling around the edges. If the crust browns too quickly, loosely cover the top with foil.

Let the pot pie rest for 10 to 15 minutes before serving so the filling can settle.