Cook the pasta for about 5 min until the shells are becoming bigger and easy to fill. I would recommend to cook them halfway so if the pack says 10 min you cook them for 4-5 min. Keep an eye on those so they are not overly cooked. (See recipe notes for some tips)
Wash the spinach and cook for 2 minutes in a frying pan with a sprinkle of water or milk if you want, a pinch of salt, one of pepper and some freshly ground nutmeg. When finish turn heat off and set aside to cool.
In a separate bowl mix the ricotta cheese with 3/4 (180g) of the grated cheese, another pinch of nutmeg and add the spinach. Mix well. Season if necessary.
Preheat oven at 180C or 350F.
Prepare the baking dish (20cm x 30cm) and start by spooning half of the tomato sauce as a first layer at the bottom of the dish. Fill the shells with roughly about a tbsp of cheese and spinach mixture until you have finished all shells and the cheese.
Arrange all filled shells over the tomato sauce in one layer. Place them pretty close to each other and squeeze some extra in between if necessary. Spoon over the shells the remaining tomato sauce.
Place the dish in the preheated oven. Bake for 30-35 minutes at 180C 350F.
Halfway through, take the dish out and sprinkle the remaining (90g or add more if you wish) of the grated cheese. Place back in the oven and bake till nice and golden brown.
When finish baking take out of the oven and serve with a light sprinkle of Parmesan on top. Enjoy!!