Clean the cremini mushrooms with a damp cloth and slice them into thin pieces.
In a medium mixing bowl, whisk together the whole eggs, egg whites, sea salt, black pepper, and garlic powder until the mixture is well combined and slightly frothy.
Heat the extra virgin olive oil in a large non-stick skillet over medium heat.
Add the sliced mushrooms to the skillet and sauté for 5 to 6 minutes until they are golden brown and any released liquid has evaporated.
Reduce the heat to medium-low and pour the egg mixture directly over the sautéed mushrooms.
Using a silicone spatula, gently push the eggs from the edges toward the center, allowing the uncooked eggs to flow to the bottom to create soft, fluffy curds.
When the eggs are nearly set but still appear slightly moist, sprinkle the crumbled feta cheese over the top and gently fold it in.
Remove the skillet from the heat immediately to prevent the eggs from becoming rubbery.
Transfer the scramble to a serving plate and top with the fresh avocado slices and chopped chives.