Crack eggs into a medium bowl and whisk vigorously for 30 seconds until fully combined and slightly frothy. Season with salt, pepper, and optional garlic powder.

Heat a non-stick skillet over medium heat with olive oil. Add spinach and cook 1-2 minutes until wilted, stirring occasionally.

Transfer cooked spinach to a plate and wipe skillet clean with a paper towel.

Return skillet to medium-low heat and add butter, allowing it to melt and coat the pan evenly.

Pour whisked eggs into skillet and let sit 30 seconds until edges begin to set. Gently push cooked edges toward center, tilting pan to allow uncooked egg to flow to edges.

When eggs are mostly set but still slightly wet on top (2-3 minutes), arrange cooked spinach over one half of omelet and sprinkle with crumbled feta.

Carefully fold empty half over filled half using spatula, creating half-moon shape. Cook 30-45 seconds more.

Slide omelet onto serving plate and serve immediately while hot.