Preheat oven to 375°F (190°C).
In a bowl, whisk egg whites with salt and pepper until slightly frothy.
Heat olive oil in a 10-inch oven-proof skillet over medium heat.
Add spinach and sauté until wilted, approximately 2 minutes.
Pour egg whites into skillet and distribute spinach evenly.
Arrange cherry tomatoes and feta cheese over the top.
Cook on stovetop for 2 minutes until edges begin to set.
Transfer skillet to oven and bake for 10 to 12 minutes until the center is set.
Garnish with chives and serve immediately.