Dutch Baby with Ham, Leeks and Sharp Cheddar

Source: USA Weekend Magazine

1 cup whole milk
4 large eggs
1/4 tsp. salt
1 cup all-purpose flour
2 T butter
1 medium leek, washed and diced (about 1 cup)
Generous 1/2 cup small-diced ham (about 3 ounces)
1/2 tsp. dried tarragon
1 cup grated extra-sharp cheddar cheese

Adjust rack to middle position and preheat oven to 425.
Whisk milk, eggs and salt in a medium bowl; vigorously whisk in flour until mostly smooth with a few small lumps; let stand a few minutes and whisk again. (Use a blender for instantly smooth batter).
Heat butter in a 10-inch skillet (a cast-iron skillet works well) over medium heat. Add leeks and ham; cook until leeks are soft, 3 to 4 minutes. Add tarragon and cook a minute or so longer. Pour milk mixture into skillet. Transfer to the oven and bake until puffed and golden, 15-17 minutes.
Without removing Dutch Baby from oven, sprinkle with cheese and turn oven to broil. Broil until very puffy and golden brown, a couple of minutes longer.
Serve immediately from the skillet.

Serves 4 to 6.