Scrape the ribs from inside half of a Portobello mushroom, using a spoon.
Place mushroom on a parchment lined baking sheet.
Bake the mushroom at 400 degrees for about 10 minutes, to release some of the liquid. Blot the liquid out of the inside of the mushroom with a paper towel or napkin.
Fill Portobello mushroom with 1/3 cup egg whites. Crack one egg into the center. Add a pinch of salt to the top.
Bake mushroom with egg filling in oven at 400 degrees, for 20 minutes, or until egg is completely cooked. You can tell if it no longer looks wet on top, and you can touch it without it feeling runny.