Preheat oven to 450°.
Combine flour, milk, eggs, and salt in a blender container. Pulse until smooth.
Heat butter in a Dutch baby pan or a large iron skillet.
Cook green onions for 3-4 minutes, or until just beginning to brown.
Pour flour mixture into pan (or skillet).
Immediately place pan in hot oven and bake at 450° for 18-20 minutes or until puffed and brown.
Top Dutch Baby with smoked salmon, capers, crumbled Boursin cheese, arugula, and a sprinkle of freshly ground pepper. (Dutch Baby will deflate and fall after being removed from oven.)