Preheat oven to 375°F/190°C. Generously butter an 11-inch (28 cm/1 ½ quart ) oval dish.

Melt the 3 tablespoons butter in a pan over medium heat. Whisk in the flour until it absorbs the butter. Cook while stirring for 2 minutes to cook out the flour taste, it will start to turn darker when ready. Stir in the garlic and rosemary, don’t burn the flour or the garlic. Slowly whisk in the milk until thickened and there are no lumps. Add half the cheddar cheese and stir until completely melted, add the rest of the cheddar and stir until sauce is smooth. Taste the sauce and add salt if needed. This will depend on how salty your cheese is. Keep warm over very low heat.

Add the half the potato slices to the baking dish, slightly overlapping in 2 layers, then top with the all of the celeriac slices, slightly overlapping. Pour over half of the sauce. Add the rest of the potato slices and pour over the rest of the sauce pushing the sauce into all the nooks and crannies. Sprinkle the top evenly with the Parmesan cheese.

Cover tightly with foil and bake for 1 hour, or until the potatoes are tender when pierced with a sharp knife. Remove foil and continue to bake for 15 more minutes until golden brown and bubbly. Time will depend on how thin the vegetables are sliced. If you would like it more browned, you can broil for a couple of minutes (or oven grill if you’re in the U.K).