Heat a 10-inch nonstick or well-seasoned skillet over medium heat. Add olive oil and when shimmering add the diced onion and red bell pepper. Cook, stirring occasionally, 3–4 minutes until softened and translucent.

Cook mushrooms and jalapeño

Add the sliced mushrooms and chopped jalapeño to the skillet. Continue cooking 3–4 minutes until the mushrooms release their moisture and begin to brown.

Wilt spinach

Add the packed spinach to the pan and cook 1–2 minutes, stirring, until wilted. Transfer the vegetable mixture to a small bowl and set aside. Wipe the skillet clean if it has excess liquid.

Make Omelette Base

In a bowl, crack the eggs and add the milk, salt, and black pepper. Whisk vigorously until the mixture is homogenous and slightly frothy, about 20–30 seconds.

Return the skillet to medium-low heat. Add the butter and let it melt and foam but not brown (about 30–45 seconds). Swirl to coat the pan evenly.

Pour the beaten egg mixture into the skillet, tilting the pan to distribute evenly. Let the eggs set at the edges for 20–30 seconds.

Using a silicone spatula, gently stir the eggs in small circles for the first 15–20 seconds to form soft curds, then stop stirring and let the top finish setting but remain slightly moist, about 30–45 seconds more.

Assemble & Finish

When the omelette surface is mostly set but still a touch glossy, sprinkle the grated cheddar evenly over one half of the omelette.

Spoon the reserved vegetable mixture over the cheese in an even layer. If using, sprinkle the crumbled cotija or feta on top of the vegetables.

Use the spatula to carefully lift the opposite half of the omelette and fold it over the filling. Press gently to seal and let cook 30–45 seconds to melt the cheese and finish cooking the center.

Slide the omelette onto a warm plate, garnish with chopped cilantro and sliced scallion, and serve with a lime wedge and hot sauce or salsa on the side. Squeeze lime over omelette just before eating for brightness.