Prep everything first: slice the mushrooms, roughly chop the spinach, crumble the feta (if using), and crack the eggs into a medium bowl. Mise en place speeds the process and ensures nothing overcooks while you hunt for ingredients.

Whisk the eggs with 1/8 tsp kosher salt and a pinch of pepper until the mixture is homogenous and slightly frothy (about 20–30 seconds). Whisking incorporates air for a lighter omelette and distributes the salt so the eggs season evenly.

Heat a nonstick 10–12-inch skillet over medium heat and add 1 tsp olive oil. Let the oil warm until it shimmers but is not smoking — this ensures even heat for the vegetables. Place the bread in a toaster or under a broiler to toast while you cook the filling so the toast finishes warm and crisp.

Add the sliced mushrooms to the pan in a single layer and sprinkle with a tiny pinch of salt (about 1/8 tsp). Sauté undisturbed for 2 minutes to allow them to brown: browning (the Maillard reaction) builds a deep, savory flavor you won’t get from steaming. Stir and continue to cook 1–2 minutes until the mushrooms are golden and their liquid mostly evaporated.

Push the mushrooms to one side, add the remaining 1 tsp olive oil to the empty side of the pan, then add the chopped spinach. Sauté briefly, 30–60 seconds, tossing until wilted — spinach cooks very quickly. Combining cooked mushrooms and wilted spinach now prevents the eggs from overcooking later.

Transfer the mushroom-spinach mixture to a plate and wipe the pan quickly with a paper towel (very important for an even omelette surface). Reduce heat to low–medium. A slightly lower heat prevents the bottom of the omelette from overbrowning while the interior sets.

Increase heat under the skillet to medium-low and add a small nonstick-friendly smear of butter or a light brush of oil if the pan is dry. Pour the beaten eggs into the warm pan and tilt to distribute evenly. Let the eggs sit undisturbed for 10–15 seconds so a thin film forms on the bottom; this helps you form a cohesive omelette and prevents sticking.

Use a heatproof spatula to gently push the set edges toward the center while tilting the pan so runny egg flows to the edges. This technique cooks the eggs evenly without flipping and yields a tender, slightly custardy interior — the hallmark of a restaurant omelette.

When the egg is mostly set but still slightly glossy on top (about 60–90 seconds after pouring), spoon the mushroom-spinach mixture across one half of the omelette. Sprinkle the crumbled feta evenly over the vegetables. Adding the filling when the eggs are just set prevents overcooking and keeps textures distinct.

Fold the empty half of the omelette over the filling using your spatula, creating a neat semicircle. Let it cook 20–30 seconds more to warm the filling through and let the feta soften slightly without fully melting — this preserves its crumbly, tangy character.

Slide the omelette onto a plate, slice the toast (butter it if you like), and season the omelette with a final pinch of flaky salt and a crack of black pepper. The contrast of the warm crisp toast with the soft omelette completes the dish.

Serve immediately with the two slices of toast per person (total 4 slices) and enjoy. Omelettes are best right off the stove when the eggs are still tender and the toast is crisp.