Preheat oven to 425 F
Place the eggs, milk, flour, baking powder, salt and herbs into a blender. Blend at high speed until smooth, or use a mixer. Set aside for 15 minutes
Using an 8-inch cast iron skillet, heat 2 tablespoons of butter over medium heat. Add onions and peppers with a pinch of salt and saute until the onions start to soften and caramelize. About 5 minutes. Transfer the onions with peppers into a bowl
Add the remaining 2 tablespoons of butter into the same skillet and place the skillet in the hot oven until the butter has melted. It takes about 1 - 2 minutes. Carefully take the skillet out and swirl the butter around to make sure the pan, including the sides, are coated
Immediately spread the onion and pepper mixture on the bottom of the cast iron skillet, pour the egg mixture on top, sprinkle with the feta cheese and olives and finish with Parmesan. Place it back in the oven and bake for 20 minutes or until puffed and the edges are dark brown.
Serve hot with fresh herbs and desired sides
Chefs notes: you can make the Dutch baby ahead of time. When ready, reheat it in a warm oven until warmed through
