Clean the cremini mushrooms and slice them thinly.
Place a non-stick skillet over medium-high heat and add the olive oil, spreading it evenly across the surface.
Add the sliced mushrooms to the skillet and sauté for 5-7 minutes until they are browned and all moisture has evaporated.
In a medium bowl, whisk together the liquid egg whites, fat-free Greek yogurt, sea salt, black pepper, and garlic powder until the mixture is smooth and slightly frothy.
Remove the mushrooms from the pan and set them aside.
Wipe the skillet if necessary and spray with a light coating of non-stick spray, then pour in the egg white mixture over medium heat.
As the edges set, gently lift them with a spatula to let the uncooked egg flow underneath.
Once the top is nearly set, spread the sautéed mushrooms and fresh chives over one half of the omelette.
Carefully fold the other half over the filling and cook for another 30 seconds to heat through.
Toast the sourdough bread slices until golden brown.
Slide the omelette onto a plate and serve immediately alongside the toasted sourdough.