Step 1In a small saucepan over medium heat, melt 1 teaspoon of the butter. Add the diced pancetta and saute, stirring occasionally, until crisp and browned, about 3 minutes. Stir in the onion and continue to saute until translucent, then stir in the garlic and diced tomatillo. Continue to cook until the tomatillo is crisp-tender, about 2 minutes. Season to taste and set aside in a warm place.
Step 2In a small bowl, beat together the eggs with one-fourth teaspoon salt and a pinch of pepper.
Step 3In an 8-inch skillet, heat the remaining tablespoon of butter over medium heat until melted, add the eggs and stir until they just start to set on the bottom of the pan. Sprinkle the cheese evenly over the eggs and reduce the heat to medium-low. Continue to cook until the top of the omelet is set and the bottom is nicely browned, 3 to 5 minutes. Fold the omelet in half and continue to cook until the cheese is completely melted, about 1 more minute.
Step 4Transfer the omelet to a serving plate and spoon over the tomatillo sauce. Sprinkle with the chopped cilantro and serve immediately.