Add the spinach into a pot with boiling water. Enough to cover the whole spinach. In large pot of boiling salted water, cook spinach for 30 seconds. Drain and place in a bowl of ice water. When cool enough to handle, drain and squeeze out as much excess water as possible.
After 30 seconds, drain the spinach and use a spoon to remove all excess water. When it is cold enough you can also use your hands.
In a medium-high heat pan, add the butter. Once it is melted, add the onion and cook for about 3 minutes, or until the onions is soft and translucid. Add garlic and cook for 1 more minute.
In the same pan, now add the milk, cooking cream, and grated parmesan. Mix it well and let it simmer until it gets creamier. Season with salt, pepper.
Finally, add the drained spinach. Mix it well. Serve it as a side dish.