Halve the Kalamata olives, julienne the sun-dried tomatoes, chop the spinach, mince the garlic, and dice the red onion. Set ingredients aside.
Beat the eggs
Crack eggs into the mixing bowl. Add milk (or water), Dijon mustard, salt, and pepper. Whisk vigorously until pale and frothy—this step helps you achieve that fluffy, restaurant-style texture.
Saute aromatics and veggies
Heat skillet over medium heat. Add the olive oil and swirl it around.
Add onion and garlic. Sauté for 1 minute until fragrant but not browned.
Stir in the spinach and sun-dried tomatoes. Cook until spinach wilts, about 1–2 minutes.
Add olives and cheese
Fold in Kalamata olives and half the feta cheese. Give everything a quick stir to distribute flavors evenly.
Pour in eggs
Tilt the skillet to spread veggies evenly. Pour beaten eggs over the veggies.
Gently swirl the pan so the eggs settle in and around the fillings.
Cook the omelette
As the edges begin to set after about 1 minute, gently push the edges toward the center, tilting pan so uncooked egg flows underneath.
When the top is mostly set but still slightly runny, sprinkle the rest of the feta on one half.
Fold and finish
Fold the omelette in half with the spatula. Let it cook for 30 seconds more.
For bubblier, browned spots: increase heat slightly for 15 seconds, watching closely.
Serve and garnish
Slide onto a plate and sprinkle with fresh parsley or dill. Enjoy immediately with a side salad, toasted bread, or fresh fruit.