Whisk eggs with chopped herbs and whole milk in a bowl and season with salt and pepper.
In a medium ovenproof nonstick skillet, melt 2 tablespoons butter over moderate heat. Add onion and saute until it is softened, about 6 minutes.
Add vermouth and increase heat to high and cook until evaporated, about 1 minute. Reduce heat to moderate and add the remaining 2 tablespoons of butter to the skillet.
Pour in the egg mixture. Carefully arrange the tomato slices on top of the egg mixture in a single layer in a pretty circle. Cook eggs without touching, until the edge just begins to set, about 4 minutes.
Place skillet in oven and bake for 30 minutes, or until it is set in the center.
Let the frittata stand for 5 minutes, then slide it onto a large plate and serve.