In a 2- to 3-quart microwave-safe casserole with a lid microwave the bacon, uncovered, at high power (100%), stirring after each minute, for 2 to 2 1/2 minutes, or until it is almost crisp. Pour off all but 1 tablespoon of the fat from the casserole and stir in the onions. Microwave the mixture, uncovered, at high power, stirring every 2 minutes, for 8 minutes, or until the onions are softened but not browned, and transfer the onion mixture to a bowl.
To the casserole add the water, the broth, and the sage, whisk in the cornmeal, a little at a time, whisking until the mixture is smooth, and microwave the mixture, covered, at high power, whisking after 3 minutes, for 5 minutes, or until it is thickened and the liquid is absorbed. Stir in the butter, 2 tablespoons of the Parmesan, half the onion mixture, and salt and pepper to taste and divide the polenta mixture between two 1 1/2-cup gratin dishes or spread it in a 3-cup gratin dish.
Top the polenta with the remaining onion mixture, spreading the onion mixture evenly, sprinkle it with the remaining 2 tablespoons Parmesan, and bake it in the middle of a preheated 400°F. oven for 15 minutes, or until the Parmesan is melted.
