Start by bringing a large pot of salted water to a boil. Add the peeled potatoes and turnips. Boil them until they're tender when pierced with a fork—around 20 minutes. Once cooked, drain the water and return them to the pot.

With the potatoes and turnips back in the pot, mix in the butter and milk. Use a potato masher or a hand mixer to create a smooth and creamy mixture. Season generously with sea salt and black pepper, then fold in chives and grated Parmesan cheese. Set this aside while you tackle the filling.

In a large skillet, cook the diced bacon over medium heat until crispy. Remove the bacon with a slotted spoon and set it aside, leaving behind the drippings. Add the chopped onions, carrots, and celery to the skillet. Cook until the vegetables are softened, around 5-7 minutes.

Once the veggies are soft, add the ground beef to the skillet. Cook until it's browned, breaking it up with a spoon. This is the time to add in the finely chopped mushrooms and thyme, stirring until mushrooms release their moisture and soften.

Next, stir in the tomato paste, all-purpose flour, chicken broth, Worcestershire sauce, and season with kosher salt and black pepper. Let this simmer for about 5 minutes, ensuring the mixture thickens nicely. Finally, fold in the crispy bacon, Dijon mustard, and smoked paprika. Remove from heat.

In a baking dish, pour the filling mixture evenly. Then, spread the creamy potato and turnip mixture over the top, smoothing it out with a spatula. You can create some texture using the back of a fork, which can give it a nice crust when baked.

Preheat your oven to 400 degrees F (200 degrees C). Place the assembled pie into the oven and bake for 30 minutes, or until the top is golden brown. The kitchen will have the most incredible aroma by now, and just wait until you dig in!