Make the stock. Combine all ingredients except pepper and salt or soy sauce in a large pot. Bring to a boil. Skim off foam that has risen to top. Immediately lower heat and simmer, uncovered, until well flavored (1 to 2 hours). Add water as necessary to keep vegetables covered. Skim as necessary. Season to taste with pepper and salt or soy sauce at end of cooking time.
Strain stock pressing vegetables with back of spoon to extract as much liquid as possible. For a thicker stock, puree vegetables in a food mill or blender, add to liquid and strain. Cool stock to room temperature, then refrigerate or freeze., Plan ahead, make this recipe and put in refrigerator to give its flavors time to meld.
Preheat oven to 400 degrees F. Heat 1 1/2 tablespoons oil in a large nonstick skillet. Add onions and garlic; cook over medium heat until deep golden brown, about 10 to 15 minutes, stirring frequently.
Sitr in sweet potatoes and parsnips. Add stock, sour cream, thyme and a little salt and pepper. Simmer until potatoes are tender and liquid is absorbed, 15 to 20 minutes. Add more salt and pepper to taste. Transfer to a 2-quart baking dish.
Sprinkle casserole with bread crumbs and drizzle with remaining 1 teaspoon olive oil. Bake casserole until stock has been absorbed and top browns, about 20 minutes.