For the stew: In heavy, large saucepan over moderately-high heat, heat 3 tablespoons oil until hot, but not smoking. Add the onion, carrot, and celery and saute, stirring occasionally, until light golden brown, about 10 minutes.
Add the beef and saute, stirring occasionally, until brown on all sides, about 5 minutes. Add wine and thyme, stir well, and bring to boil.
Add the tomatoes, salt, and pepper, then lower heat to moderately low, cover, and simmer, stirring every 15 minutes, until the beef is tender, about 2 hours.
For the polenta: While the beef is simmering, make the polenta. Pour the olive oil into a large serving bowl and swirl to coat. Set aside.
In heavy, large pot over moderately-high heat, bring stock to a boil. Lower the heat to moderate and gradually whisk in the polenta, stirring constantly. Continue cooking, stirring constantly, until polenta thickens and pulls away from sides of pan, about 5 minutes. Transfer the polenta to an oil-coated serving bowl and keep warm. Let the polenta sit for 5 to 10 minutes so it can set up.
When the beef is tender, use tongs to remove the tomato skins, if desired and the thyme sprigs.
Turn the polenta out of the bowl, slice, and serve alongside the stew.