In a small skillet, heat the oil over medium heat and add the pancetta, cooking until crisp.
Add the garlic and cook until golden. Toss in the spinach and cook until wilted. Drain off any liquid and set aside until ready to use.
In a small bowl whisk together the eggs, milk, salt and pepper. Melt the butter in a 10-inch nonstick skillet over medium heat.
Pour in the egg mixture and tip the skillet to coat evenly with the egg.
As the eggs cook, periodically tilt the skillet, lifting the cooked portion to allow the uncooked egg mixture to slide beneath.
Once the eggs are cooked, but the surface is still shiny, layer the spinach, chicken and cheese over one half of the omelet.
Slide the omelet out of the pan, filling side first, onto a warm plate. While sliding the omelet out of the pan, lift up the pan to tip the plain side over the filling. Serve immediately.
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