Scrub potatoes under cold running water. Peel. Cut each potato in half lengthwise and place on cutting board, cut side down. Cut 1/8-inch thick slices in each potato half, cutting to within ¼ to ½ inch of bottom, but not completely through.
Arrange potato halves, cut side up, in a buttered 9×13-inch or 10×12-inch baking pan and set aside.
In small skillet over medium heat, heat olive oil and add onion. Cook, stirring occasionally, until onion is caramelized, about 10 minutes. Spoon onions over potatoes in pan. Pour in chicken stock and add thyme, salt and pepper. Cover with waxed paper and then with foil.
Bake in preheated 350° oven until tender but not mushy, about 20 to 30 minutes. Remove foil and waxed paper and sprinkle tops of potatoes with Parmesan cheese. Return to oven just until cheese begins to brown, about 5 minutes.
Nutritional Analysis per serving: 96 calories, 3 g protein, 0.8 g fat (8% of calories from fat) 0.3 g saturated fat, 19.44 g carbohydrates, 2 mg cholesterol, 171 mg sodium.