Melt the butter in 2 preheated 12-inch (30 ml) non-stick skillets. Pour half the eggs into each skillet. Cook over medium heat for 1 to 2 minutes, using a spatula to pull apart the omelets and let the uncooked eggs run to the bottom. When the edges (not the centre) of the omelets are almost done, sprinkle 1 side of each with half of the filling and half oh the Gruyère. Using the spatula, fold the omelets over the filling and slide onto plates.