1. In a 5- to 6-quart pan, blend polenta, chicken broth, 1 cup water, and evaporated nonfat milk. Stir over high heat until mixture boils, then reduce heat and simmer, stirring often, until polenta is smooth to taste, about 15 minutes. Stir in the shredded parmesan cheese, minced fresh chives or green onions, sage, and salt and pepper to taste.
2. While polenta cooks, in a deep bowl, with a mixer on high speed, whip 4 large egg whites until they hold soft peaks.
3. Remove polenta from heat and stir in 2 large egg yolks and about 1/3 of the whipped egg whites, then fold in remaining whites.
4. Scrape mixture into a lightly oiled shallow 1 1/2-quart casserole (8- by 11-in. oval) and spread level.
5. Bake spoon bread in a 375° oven until well browned at the edges and lightly browned on top, about 40 minutes (about 30 minutes in a convection oven). Spoon portions onto plates.