1. Sauté the Vegetables: Heat one tablespoon of butter in your favorite non-stick skillet over medium heat. Add the sliced mushrooms. Resist the urge to toss them immediately; let them sit and develop a deep golden-brown crust. That color is flavor. Toss in the spinach once the mushrooms are caramelized. Stir just until the leaves wilt into vibrant ribbons. Remove the mixture from the pan and set it aside.
2. Whisk with Intent: Crack your eggs into a medium bowl. Add the sea salt and pepper. Whisk them thoroughly—you want to break up the proteins completely so the whites and yolks are a uniform, pale yellow. If you see streaks, keep whisking.
3. The Gentle Cook: Return the pan to the stove over medium-low heat and add the remaining butter. Once it foams, pour in the eggs. Tilt the pan in a circular motion to ensure an even coating. Using a heat-resistant spatula, gently lift the edges of the setting eggs, tilting the pan so the runny, uncooked egg flows into the empty spaces. This is how you achieve that coveted, velvety texture.
4. Assembly: When the eggs are set but the center still looks slightly glossy, spread your sautéed mushrooms and spinach over one half. Sprinkle the crumbled feta over the top. Slide your spatula under the empty half of the omelet and fold it over the filling. Give it a moment to seal, then slide it onto a warm plate. Garnish with a generous scattering of fresh chives.

I get asked all the time if an omelet can be made ahead. While the absolute best result is eating it straight from the pan, you can streamline the process. Sauté your mushroom and spinach mixture up to two days in advance and keep it in an airtight container in the fridge. When you're ready for breakfast, simply warm the veggies, whisk your eggs, and proceed with the cook. I do not recommend storing a fully cooked omelet, as the texture of the eggs changes significantly upon reheating.