Step 1

Place a rack in middle of oven and place a medium ovenproof skillet, preferably cast iron, on rack. Preheat oven to 425°. Place onion in a small bowl; squeeze in juice of ½ lemon and add a big pinch of salt; set aside. Slice remaining ½ lemon into wedges and set aside for serving.

Step 2

Blend eggs and egg white in a blender on high speed until very frothy, about 1 minute. Add garlic, spinach, milk, and 1 cup herbs and blend on high speed until well combined and very green. Add flour, cornstarch, and ¾ tsp. Diamond Crystal or ½ tsp. Morton kosher salt and season with lots of pepper; blend to combine.

Step 3

Carefully remove skillet from oven. Add butter to pan and swirl to coat bottom. Immediately pour batter into pan and bake pancake until puffed and brown around the edges, 20–25 minutes. (It will start to deflate as soon as it comes out of the oven, but don’t worry, that’s normal.)

Step 4

Top Dutch baby with smoked salmon, onion, and more herbs; season with pepper. Serve with reserved lemon wedges for squeezing over.