Preheat your oven to 325°F (163°C).
In a large Dutch oven, heat 2 tablespoons of oil over medium-high heat. Sear the seasoned beef chunks on all sides until deeply browned, then remove and set aside.
Reduce heat to medium. Add the sliced onions to the pot with a pinch of salt. Cook, stirring frequently, for 15-20 minutes until the onions are soft and caramelized golden brown.
Add the mushrooms, carrots, and garlic. Sauté for another 5 minutes until fragrant.
Pour in the red wine to deglaze the pan, scraping up any browned bits from the bottom. Allow the wine to reduce by half.
Stir in the beef broth and fresh thyme. Return the beef (and any resting juices) to the pot. The liquid should mostly cover the meat.
Cover the Dutch oven and transfer it to the preheated oven. Braise for 3 to 3.5 hours, or until the beef is fork-tender and shreds easily.
While the beef cooks, boil the potatoes in salted water until tender. Drain and mash with butter and heavy cream until smooth. Keep warm.
During the last 10 minutes of cooking, arrange baguette slices on a baking sheet. Top with shredded Gruyère cheese and broil until bubbly and golden.
Remove beef from the oven. Shred the meat slightly with two forks directly in the gravy.
Serve the French Onion Braised Beef over a bed of mashed potatoes, topped with a piece of cheesy Gruyère toast and garnished with fresh parsley.