Preheat oven to 450F.
Place the spinach on a plate and microwave on hight for 6 minutes to defrost. Heat a medium skillet over medium-high heat with the EVOO and the butter. Once the butter melts, add the onions, garlic, thyme, salt, and pepper and cook, stirring frequently, for 4 to 5 minutes.
While the onions are cooking, place the defrosted spinach in a clean kitchen towel and squeeze out the excess liquid. Add the spinach to the onions, break it up with a spoon, then add the chicken stock, heavy cream, and a little nutmeg. Bring it up to a simmer and cook until it's creamy and thick, a couple of minutes. Transfer the spinach mixture to a small baking dish; Rachael uses her 8x11 oval dish. With the back of a spoon, create four divots in the spinach. Crack one egg at a time into a shallow cup or ramekin, then gently slide an egg into each divot in the spinach. Season the eggs with a little salt and pepper, sprinkle with the Gruyere cheese, and bake 10 minutes for runny yolks (15 minutes for firmer eggs).
Serve alongside Bacon and Tomato Salad with Garlic Croutons.