Preheat your oven to 375°F (190°C).
In a medium bowl, whisk together the egg whites, Greek yogurt, sea salt, black pepper, and garlic powder until the mixture is smooth and slightly frothy.
Heat the olive oil in an 8-inch oven-safe non-stick skillet over medium heat.
Add the finely diced red onion to the skillet and sauté for 3-4 minutes until translucent and fragrant.
Add the baby spinach to the skillet and cook just until wilted, which should take about 1-2 minutes.
Pour the egg white mixture into the skillet over the vegetables, stirring gently to distribute the spinach and onions evenly.
Sprinkle the crumbled feta cheese evenly over the top of the egg mixture.
Transfer the skillet to the preheated oven and bake for 12-15 minutes, or until the eggs are set and the edges are lightly golden.
While the frittata bakes, toast the slice of sprouted grain bread until crisp.
Carefully slide the frittata out of the skillet onto a plate, slice into wedges, and serve immediately alongside the warm toast.