Preheat the oven to 400F/200C and grease 6 holes of a muffin pan. Roughly chop 6 of the mushrooms and halve the remaining 3 lengthways. Roughly chop the spinach.
Drizzle the olive oil into a small frying pan and sautee the mushrooms and spinach for 5 minutes.
Set the halved mushrooms to one side. Mix together the remaining roughly chopped mushrooms, spinach, eggs, milk, cheese, salt and pepper. Evenly divide the mixture between the greased muffin holes and top each one with half a mushroom.
Place the egg cups into the oven and bake for 20 minutes, until the tops are puffy and golden brown.