Preheat the oven to 200 degrees Celsius (390 degrees Fahrenheit)
Remove the stem and gills from each of the portobellos. Place them face up on a baking sheet or large oven proof pan and lightly brush each mushroom with half a tablespoon of oil and sprinkle with salt. Bake for 12 minutes. Once cooked, remove from the oven and set aside.
While the mushrooms are cooking, separate each of the eggs, combining the whites in a bowl and leaving the yolks in the shell. You will have one extra yolk which you can throw away, or alternatively add in with the egg whites.
Gently whisk the egg whites and almond milk and season with salt and pepper. On low heat, place half a tablespoon of olive in a medium skillet. Once hot add chopped spinach and cook for a few minutes until the spinach begins to wilt and then add in the egg whites. Allow the whites to sit for 1-2 minutes before stirring with a wooden spoon. Continue to cook for 2-3 minutes until the egg whites have set but are still moist. Remove from the heat.
Turn the oven off.
Fill each of the mushrooms with the egg white mixture and create a small hole in the centre. Gently place a yolk into the centre of each of the mushrooms. Cover the pan with a lid or alternatively place a sheet of tin foil over the baking sheet so all 4 mushrooms are covered. Place the tray back into the oven which is turned off and allow to sit for 3 to 4 minutes until the yolk is just set.
Remove from the oven and serve each portobello with chopped fresh chives and any other toppings of choice such as bacon, chopped tomatoes, avocado or salsa. Enjoy!