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.

In a large skillet over medium heat, melt butter. Add onion and cook until soft, 5 minutes. Add garlic and cook until fragrant, 1 minute more.

Add milk, heavy cream, and cream cheese to skillet. Simmer until cream cheese is melted. Season with salt, pepper, and a pinch of cayenne.

Add spinach and Parmesan and stir to combine.