Preheat a large nonstick skillet over medium-high heat.
Brush oil on all sides of beef and rub evenly with seasoning. Add beef to skillet and cook until well-browned on all sides. Place beef on top of onions in slow cooker. Place carrots and mushrooms around beef.
Return skillet to heat. Add wine and cook, stirring to scrape up browned bits on bottom of skillet. Pour over beef and vegetables in the slow cooker.
Cook, covered, on LOW 4 hours or until an instant-read thermometer registers 145°F. Transfer beef to a cutting board and let stand 10 minutes.
Meanwhile, add thyme and brown sugar, if using, to vegetables and cooking liquid. Stir to combine.
Thinly slice beef across the grain, and place on a serving platter.