1 medium onion, sliced thinly into half-moons (about 1 cup)
8 ounces sliced mushrooms (about 3 cups)
2 tablespoons minced fresh chives
1/2 cup grated Parmesan cheese (about 2 ounces)
01Preheat oven to 425 F. In a blender or food processor, combine eggs, milk, flour, 1/2 teaspoon salt, and pepper. Blend for about 30 seconds, scraping down the sides of the blender if necessary. The mixture should be smooth and liquid. Let the batter rest for at least 15 minutes.
02In a large pan over medium heat, heat 1 tablespoon of butter. Add the onion and cook, stirring often, until translucent and starting to brown, approximately 3 minutes. Add the mushrooms to the pan and cook until they begin to brown and the moisture from the mushrooms has mostly evaporated, about 6 minutes. Deglaze the pan by adding a couple of teaspoons of water and scraping up the brown bits.
03Remove skillet from heat and season the mushrooms and onions with the remaining 1/2 teaspoon salt. Stir in chives and set mushroom-onion mixture aside.
04While the vegetables cook, place a 10-inch cast iron skillet in the heated oven. When ready to make the pancake, place the remaining 2 tablespoons of butter in the skillet and put it back in the oven to melt, about 1 minute. With an oven mitt, remove the skillet and tilt the pan so butter coats the bottom and sides.
05Add the mushroom-onion mixture to the pan, followed by the batter. If necessary, tilt the pan so the batter spreads evenly. Sprinkle Parmesan on top.
06Place the skillet back into the oven and bake for 15 to 20 minutes, until the Dutch baby is puffed and golden brown. Remove from the oven and serve immediately.