Preheat oven to 425 degrees. In the meantime, grill the portobello mushroom caps. To do this, I just popped the mushrooms onto a prepped indoor grill for about 10 minutes, until the mushrooms became thoroughly cooked. Allow to cool. Place the ham slice on the indoor grill next, cooking for about 2-5 minutes, or until the ham becomes crispy. Place egg whites, pepper, thyme, and salt in a prepared individual-sized ramekin, then bake for roughly 10 minutes. Remove as soon as the eggs become fully cooked. The egg whites should come out of the ramekin in a nicely formed patty. To prepare the breakfast sandwich, use the grilled portobello mushroom caps as a bun, then layer with the lettuce, tomato, egg whites, and ham. Go nuts with this— mix and match toppings and see what suits your taste!
*** Note: the egg whites could easily be cooked on the stovetop as well, but I personally do not recommend this. Baking the egg whites allows them to form a better patty shape for easier eating. But, if you don’t mind a messy breakfast sandwich or if you’re crunched for time, then cooking the egg whites on the stovetop is a viable option as well!