Start by peeling and cubing your potatoes and rutabaga. In a large pot, bring salted water to a boil, then add the vegetables. Cook until fork-tender, about 15–20 minutes, then drain and keep warm.

In a mixing bowl, combine the drained potatoes and rutabaga while warm. Add softened butter, kosher salt, freshly ground pepper, and heavy cream to taste. Mash until smooth and creamy, then set aside.

Heat olive oil in a large skillet over medium heat. Add diced onion, sautéing for 3–5 minutes until softened. Add diced celery and carrots, cooking for an additional 5–7 minutes.

Stir in sliced mushrooms, cooking until tender and moisture is released, about 4–5 minutes. Add minced garlic and fresh thyme, stirring for 1 minute until aromatic.

Mix in tomato paste and Worcestershire sauce. Sprinkle all-purpose flour over the mixture, stirring until incorporated. Slowly add vegetable broth while stirring, bringing to a gentle boil to thicken for 3–5 minutes. Fold in lentils and chopped parsley.

Transfer the filling into a casserole dish, spreading evenly. Spoon the creamy root vegetable mash on top, smoothing it out. Bake at 425°F for 15–20 minutes until golden and bubbling. Garnish with fresh parsley before serving.