Cook the pasta as usual. Defrost the spinach beforehand or briefly defrost it in the microwave. Dice the onions and garlic.
Now heat some oil or butter in a saucepan. When the fat is hot enough, sauté the onions and garlic. Then add the spinach, butter and flour over medium heat. The flour is used for binding and can also be replaced with light-colored sauce binders. Bring to the boil briefly, then deglaze with the cream and stir everything well. Bring the sauce to the boil again and then stir in the cheese. Season again and wait until the cheese is completely melted.