Combine chicken broth, potatoes, rutabagas, parsnips, cloves, bay leaf, and thyme in a large pot. Bring to a boil. Reduce heat, and cover partially. Simmer until vegetables are very tender, about 30 minutes. Drain.
Transfer vegetables to large bowl. Add 1/2 cup butter or margarine . Use an electric mixer, beat mixture until mashed but still chunky. Season with salt and pepper. Transfer mashed vegetables to a buttered 13 x 9 x 2 inch baking dish.
Melt remaining 1/4 cup butter or margarine in a heavy large skillet over medium-high heat. Add sliced onions. Saute until beginning to brown, about 5 minutes. Reduce heat to medium-low. Saute until onions are tender and golden brown, about 15 minutes. Season with salt and pepper. Spread onions evenly over mashed vegetables. Casserole can be prepared up to 1 day ahead. Cover and refrigerate.
Preheat oven to 375 degrees F (190 degrees C). Bake, uncovered, for 25 minutes, or until heated through and top begins to crisp.