Start with portabello caps (without stems), one per person. In our world, they were filled with (in this order) dijon mustard, shredded spinach, diced purple onion, cheese.
I recommend shredded cheese, but all we had on hand were slices. Shredded melts into the onions and spinach and is easier to cut after it's baked.
Throw them into a hot oven (400F) with a dish of baked eggs for twenty mintues. When they're done they look something like this:
Baked eggs can be as simple or as complicated as you like. We like simple. This is a 9x12 pan filled with tomato sauce - we use what was canned last summer, but anything that's chunky and flavourful is excellent. Fill the pan with sauce and then use a spoon to creat little wells (that's why chunkier is better) and drop one raw egg into each well. Bake alongside your mushrooms - after twenty minutes it should look like this: