In a medium ovenproof nonstick skillet, melt 2 tablespoons of the butter over moderate heat. Add the chopped onion and cook, stirring occasionally, until it is softened, about 6 minutes. Add the vermouth, increase the heat to high and cook until evaporated, about 1 minute. Reduce the heat to moderate and add the remaining 2 tablespoons of butter to the skillet. Pour in the egg and herb mixture. Neatly arrange the tomato slices on top of the eggs in a single layer. Cook the eggs undisturbed until the edge just begins to set, about 4 minutes.