In a large saucepan, saute onions in olive oil until translucent, 3 to 5 minutes. Add carrots and parsnips, stir to combine. Add tomatoes and stock, combine well. Bring to a boil, then reduce to simmer.
Add bouquet garni, turnips, butternut squash, and salt and pepper to taste. Stir to combine. Cover and cook until vegetables are fork-tender, about 25 minutes.
Add turnip greens and fennel. Cook until greens are wilted, 3 to 5 minutes. Adjust seasoning by adding salt and pepper to taste. Divide the polenta among six bowls, and serve immediately, topped with the fall ragout.