Preheat the oven to 375 degrees F. In a large bowl, combine the ground beef, chopped mushrooms, thyme leaves, egg, breadcrumbs, salt, and pepper. Mix until evenly blended.
Shape the mixture into a loaf about 8 inches long. Lay the bacon slices side by side on a work surface, slightly overlapping, then place the loaf in the center and wrap the bacon around it.
Transfer the wrapped loaf to a baking sheet. Bake for 45 to 50 minutes until the internal temperature reaches 160 degrees F and the bacon is crisp.
While the meatloaf bakes, place the cubed celery root in a large pot and cover with water. Bring to a boil and cook for 15 to 20 minutes until tender.
Drain the celery root, then return it to the pot. Add the butter and milk. Mash until smooth. Season with salt and pepper to taste.
Let the meatloaf rest for 5 minutes after baking, then slice and serve with the mashed celery root.