Arrange a rack in center of oven; preheat to 325°. Pat beef dry with paper towels. Using a spice grinder, mortar and pestle, or zip-top bag and a rolling pin, pulse or crush onion soup mix, pepper, and 1 Tbsp. salt until large pieces of soup mix are broken down. Season beef all over with seasoning mix.

In a large, high-sided, enameled cast-iron or stainless steel skillet over medium heat, heat 1 Tbsp. oil, swirling to coat pan. Cook beef until just starting to brown, 3 to 4 minutes per side. Transfer to a plate.

In same skillet over medium heat, heat remaining 1 Tbsp. oil. Add onions; season with remaining ½ tsp. salt. Cook, stirring occasionally, until onions are starting to brown, about 10 minutes. Add wine and cook, stirring frequently, until liquid is slightly reduced, 2 to 3 minutes. Pour in broth and return beef to skillet. Add thyme and bring to a simmer over medium-high heat. Cover skillet with a lid or foil and transfer to oven.

Roast beef, turning halfway through, until meat easily pulls apart with a fork, 2 hours and 45 minutes to 3 hours.

Transfer beef to a cutting board. Remove and discard thyme. Return skillet to medium heat and bring sauce to a simmer. Cook, stirring occasionally and skimming fat off surface with a ladle, until liquid is reduced by about half, about 15 minutes. Shred beef into large pieces with a fork and return to skillet.

Heat broiler to high. Top skillet with cheese. Broil, watching closely, until cheese is melted and starting to brown in spots, about 5 minutes.

Top with more thyme. Serve with sliced baguette alongside.