Peel and cube both the potatoes and rutabaga. Divide into separate saucepans and cover with cold water. Season both lightly with a generous pinch of kosher salt.

Bring them both to a boil over high heat, then reduce to a simmer and cook until fork tender, about 15-20 minutes.

Drain the vegetables and keep warm while you make the filling.

Use a potato masher to roughly mash both the rutabaga and potatoes until almost smooth.

Add the rutabaga to the potatoes along with the butter, 1 tsp kosher salt, and the ½ tsp pepper and mash until smooth.

Add 2 tbsp of the heavy cream and mash together. Adjust consistency with more cream if necessary.

Preheat the oven to 425°F (220°C).

Place a wide skillet or shallow braiser over medium heat. Add the olive oil and the diced onions, seasoning with a pinch of kosher salt.

Sauté for 2 to 3 minutes until the onions begin to soften.

Add the diced carrots and celery, reducing heat to low. Sauté for 5 to 7 minutes until they start to sweat.

Add the sliced mushrooms and cook for another 4 to 5 minutes until tender.

Add the minced garlic and chopped thyme, sautéing for another minute until fragrant.

Stir in the tomato paste, Worcestershire sauce, 1 tsp kosher salt, and the ½ tsp pepper, cooking briefly to remove raw tomato paste flavor.

Sprinkle the flour over the vegetable mix and stir until combined.

Slowly pour in the vegetable broth while stirring to prevent lumps. Bring to a gentle boil and cook until slightly thickened.

Add the drained canned lentils and cook for another 2 to 3 minutes.

Stir in the chopped parsley, taste, and adjust seasoning with kosher salt and pepper as needed.

Transfer the filling to a shallow casserole dish or leave in the braiser. Spoon the mashed root vegetable topping over the filling.

Bake for 15 to 20 minutes until the filling is bubbling and the top is lightly browned.

Let the pie rest for 5 minutes, then garnish with parsley before serving.