Combine the cream and onion in a small saucepan over medium heat and bring to a simmer. Let cook until the cream has thickened and reduced by half, and the onion is soft, about 10 minutes.
Heat a large pot or Dutch oven over medium heat and add about a quarter of the spinach to the dry pot. Cook, stirring occasionally, until wilted. Add more spinach to the pot and repeat until all of the spinach is wilted.
Set a strainer in the sink and transfer the spinach to the strainer. Drain off the excess liquid and return the spinach to the pot. Add the reduced cream mixture, season well with salt and cayenne pepper, and stir to combine. Serve immediately.