Heat oven to 400 degrees F. Pierce potatoes with a fork. Place on paper towels in microwave. Microwave on HIGH for 16 minutes, turning over halfway through.
Meanwhile, heat oil in a large nonstick skillet over medium heat. Add onion and cook 5 minutes. Stir in half the spinach and cook down slightly. Add remaining spinach and cook until all is wilted, about 3 minutes. Season with 1/4 teaspoon of the salt and the pepper. Remove skillet from heat; transfer spinach mixture to a large bowl and cool slightly.
Line a small baking sheet with aluminum foil; place potatoes on foil. Carefully cut open potatoes; sprinkle with remaining 1/4 teaspoon salt.
Beat the egg white to stiff peaks. Once the spinach mixture has cooled slightly, stir in egg yolk and all but 2 tablespoons of the feta cheese. Fold the egg white into the spinach mixture. Divide spinach mixture evenly among potatoes (about 1/2 cup in each). Sprinkle all with remaining cheese.
Bake at 400 degrees F for 10 minutes. Increase heat to broil. Run potatoes under broiler for 2 minutes.