Preheat the oven to 200 ° C, grease a baking dish.
Thaw the spinach, put it in a saucepan and bring to the boil. Add the cream to the spinach, season with salt and pepper.
Chop the onion and garlic. Fry both briefly in a second saucepan, deglaze with cream and milk. Let simmer, stirring well. Season with salt, pepper and a large amount of thyme.