Heat oven to 425 degrees F. In large bowl combine potatoes, carrots, parsnips, and fennel; toss thoroughly with 2 tablespoons oil, melted margarine, 1 teaspoon salt, and pepper. Divide vegetables evenly between two 13- x 9- x 2-inch roasting pans Place pans on separate shelves in oven. Bake 40 to 50 minutes until vegetables are tender and golden brown, turning occasionally with wide metal spatula. Halfway through cooking time, switch pans from one oven shelf to the other and add garlic, half to each pan, tossing in well. Transfer vegetables to one large serving bowl; toss gently but thoroughly with chopped thyme, vinegar, and additional 1 tablespoon oil if using (see note), and additional salt to taste. Serve garnished, if desired, with thyme sprigs.