Peel celeriac and slice thinly. 
Using a round 8-diameter ramekin cup cup the slices in order to have 16 perfect circles. Steam the rounds at 100° for 5 minutes, then cool in water. Shred the rest of the celeriac with a knife and brown in a pan with 10 g butter for 5 minutes, adjust for flavor and pour into a bowl.  

Pour in ricotta and 80 g Parmesan cheese, mix well and add a pinch of nutmeg and salt. Pour into a pastry bag and store in the refrigerator. Saute spinach in a hot pan with 5 g butter and cook for 1 minute, adjust for flavor and let cool. Drain the celeriac slices and pat dry with a paper towel. Butter a glass baking dish and lay the first slice of celeriac in it. Dress the first circle with the ricotta and lay a clump of spinach on top.

Scratch the parmesan rind on the outside and cut it into 2x2 cm cubes, lay them on a glass tray and bake in microwave mode at maximum power for about 1 minute.