Set oven to 425ºF and set a rack in the middle. Set a 12" cast iron skillet (or several smaller ones) in the oven to preheat at the same time.
Add the flour, salt, pepper and a 1/2 cup shredded cheddar to a large bowl and whisk to combine.
In a separate bowl, whisk the eggs, milk, chives and mustard powder together until no streaks of egg white remain.
Pour the wet ingredients into the dry and stir until just combined. A few lumps are fine.
Add butter to the hot skillet in the oven. Allow it to melt and foam until just begins to smell nutty, about 3 to 5 minutes.
Pour the batter into the pan(s). The batter should come up no higher than 3/4 of the way up the side. Bake for 15 minutes, then sprinkle remaining cheese evenly over the top.
Continue to bake until puffed and golden, about 10 minutes more.
To serve, top with a few slices of prosciutto or ham, some thinly sliced radishes and lightly dressed greens.