Clean and thinly slice the cremini mushrooms.
In a medium bowl, whisk together the whole egg, egg whites, Greek yogurt, sea salt, and black pepper until the mixture is uniform and slightly frothy.
Heat the extra virgin olive oil in a non-stick skillet over medium heat.
Add the mushrooms to the skillet and sauté for 4-5 minutes until they have released their moisture and turned golden brown.
While the mushrooms are cooking, toast the sourdough bread to your desired level of crispness.
Lower the skillet heat to medium-low and pour the egg mixture over the mushrooms.
Using a silicone spatula, gently push the eggs from the outer edges toward the center, creating soft curds.
Continue cooking for 2-3 minutes until the eggs are just set but still appear moist and velvety.
Remove the skillet from the heat immediately to prevent overcooking.
Place the toasted sourdough on a plate, top with the mushroom scramble, and garnish with crumbled feta cheese and chopped fresh chives.