In a heavy 4-quart pot over mediumlow heat, warm the butter and oil. Add the onion, fennel, and parsnip. Cook, stirring frequently, to soften the vegetables without browning, about 7 minutes. Add the cauliflower, turnips, 3-1/2 teaspoons salt, broth, and sugar. Bring to a boil over high heat, then lower the heat, cover, and simmer until the vegetables are soft, 25 minutes. Turn off the heat and let sit, covered, for a few minutes.