Cut 1/2 inch from the stem end of each onion, leaving the root end intact, and with a melon-ball cutter scoop out the centers, reserving 1 1/2 cups of the centers and leaving 1/4-inch shells. In a kettle of boiling salted water boil the onion shells gently for 5 minutes and let them drain, inverted, on paper towels. Chop fine the reserved onion. In a heavy skillet cook the bacon over moderate heat until it is crisp, transfer it with tongs to paper towels to drain, and crumble it. Pour all but 1 tablespoon of the fat from the skillet, add the butter, and heat the mixture over moderately high heat until the foam subsides. Add the potatoes, peeled and cut into 1/4-inch dice, the chopped onion, and the bell pepper and saute the vegetables, stirring frequently, for 4 to 6 minutes, or until the potatoes are golden brown and just tender. Remove the skillet from the heat and stir in the parsley, the thyme, 3/4 cup of the Cheddar, and salt and pepper to taste. Sprinkle the onion shells with salt and pepper, divide the potato mixture among them, and put the onions in a shallow 15 1/2- by 10 1/2-inch baking pan. Add 1 cup water to the pan and bake the onions in a preheated 350°F. oven for 30 minutes. Top the onions with the remaining 1/4 cup Cheddar and bake them for 10 to 15 minutes more, or until the cheese is melted. Serves 6 as a side dish or luncheon entree. Gourmet September 1990