Preheat oven to 350°. Butter a shallow 3-qt. baking dish (about 10 by 11 in.) and arrange pears in an attractive pattern over the bottom. (Fruit will rise to the top as batter bakes.)
Put milk, eggs, granulated sugar, vanilla, orange zest, nutmeg, salt, and flour (in this order) in a blender and whirl until very smooth and frothy, 1 minute. Pour batter over pears.
Bake until clafoutis is well browned and a little puffed, about 50 minutes.
Let clafoutis cool on a rack about 15 minutes. Serve warm, sprinkled with powdered sugar.