Preheat your oven to 180C/350F and grease an 8×10-inch (20x 25 cm) casserole pan. Peel the celeriac and grate it. Place in a clean kitchen towel, squeeze to release excess moisture, then place into a bowl.
Place a large skillet over high heat. Finely dice the bacon and add it to the skillet. Cook for 2-3 minutes, stirring constantly, until crispy. Remove 1-2 tbsp of the cooked bacon and set aside.
Add the grated celery root to the pan with the bacon. Stir for a minute just to coat everything with bacon fat. Transfer to the casserole.
Crack the eggs in a bowl. Add the heavy cream and season with salt and pepper. Whisk until combined.
Pour the eggs into the casserole with the other ingredients. Stir to distribute the mixture evenly. Sprinkle the cheese on top and top with the cooked bacon you reserved earlier.
Bake for 25-30 minutes until cooked through. If the cheese starts browning quickly, cover the casserole with aluminum foil. Take the baking dish out of the oven and let it cool for 10 minutes.
Slice the casserole into 4. You can garnish with your choice of herbs. Serve immediately.