Add the diced potato to a small saucepan with a pinch of kosher salt. Cover with water and cook over medium-high heat until fork-tender, about 8-10 minutes.

Pass the potatoes through a fine mesh strainer to drain. Set aside.

Meanwhile, cook lentils according to package directions, or until tender and cooked through.

Preheat oven to 425° F / 220° C. Spray a deep, 9-inch round baking dish with olive oil or nonstick spray.

Add the olive oil to a stock pot or large saucepan over medium-high heat. Add the onion and cook, stirring occasionally, until soft and slightly translucent, about 5 minutes.

Add the carrots, celery, and garlic and cook until the vegetables begin to soften, about 5 minutes more.

Sprinkle the flour over the vegetables and use a rubber spatula to toss. Cook for 1-2 minutes, or until flour smells nutty.

Add the water, milk, onion powder, dried rosemary, dried thyme, kosher salt, ground black pepper, and Worcestershire sauce or Marmite. Cook over medium heat, stirring constantly, until thickened, about 4 minutes.

Stir in the frozen mixed vegetables, potatoes, and lentils.

Remove from heat and spoon filling mixture into prepared baking dish.

Unroll pie crust and place it over the filling mixture. Use a sharp paring knife to trim away any excess, allowing a 1/2-inch overhang on all sides.

Fold the pastry over the edges of the dish. Crimp with a fork.

With a pastry brush, brush the beaten egg over the pastry.

 

With a sharp paring knife, cut 4-5 small slits in the pastry for ventilation.

Place baking dish on a rimmed baking sheet.

Bake in preheated oven for 25 – 30 minutes, or until deeply golden brown. Let stand on a wire rack for 5-10 minutes prior to serving. Serve hot.

Wrap leftover pot pie in foil transfer to an airtight container. Refrigerate for up to 5 days.