Start this recipe by placing the Portobello mushrooms onto a baking tray and remove the stalk. Season well with salt and pepper and drizzle with a little olive oil. Bake for 10 minutes or until soft. Meanwhile, make the sauce by frying the butter and onion together for 3 minutes till soft then stir in the flour and cook for a minute. Add in the milk and cream and mix thoroughly to create a sauce. Add in grated nutmeg, the cheeses and the chopped spinach and mix well. Next, heat a pan with water and drop in the vinegar. Bring to the boil, drop down the temperature to a simmer and poach the 4 eggs till cooked. To serve, place 2 Portobello mushrooms onto 2 serving plates. Fill each mushroom with the creamed spinach and place a poached egg on top of each mushroom. Garnish with a few salad leaves and serve immediately.