To make the vegetarian shepherds pie, start by making the vegetable base. Add 4 tablespoons butter to a large saute pan over medium-high heat. When melted, add in your mushrooms and cook until they release all their liquid and the liquid evaporates, stirring occasionally. Season with salt and pepper. Remove from heat and set aside.

Add the remaining 2 tablespoons butter to a large pot or Dutch oven over medium-high heat. Once melted, add in your chopped onions and leeks. Cook for 3 to 5 minutes or until translucent.

Add in your garlic and celery. Cook, stirring, until they are fragrant.

Stir in your lentils, tomato paste, sugar, soy sauce, thyme, rosemary, and bay leaf. Fill the pot with the vegetable broth and carefully stir to combine.

Bring to a boil, lower the heat to a simmer, and let simmer for 30 minutes. Drain any excess liquid. Stir in your reserved mushrooms and season with salt and pepper. Remove your bay leaf and discard.

While waiting for the lentils to cook, peel your celery root, parsnips, and carrots and cut into small pieces.

Add to a large pot filled with cold water, then bring the pot to a boil.

Cook for 25 to 30 minutes, or until the vegetables are fork tender. Drain and mash.

Add the mashed vegetables back to the large pot along with the salt, Greek yogurt, and butter.

Let the butter melt, then stir until all ingredients are well combined.

Preheat your oven to 350 degrees F. Lightly grease a 9 x 13 inch baking dish. Spoon the lentil mixture into the dish, spreading evenly.

Top with the root vegetable topping, spreading evenly.

Add to your preheated oven and bake for 30 to 40 minutes. Sprinkle with chopped parsley and serve.