Preheat the oven to 325°F. Season the chuck roast generously with salt and pepper on all sides.

Heat the oil in a large Dutch oven over medium-high heat. Sear the roast on all sides until deeply browned, about 4-5 minutes per side. Remove and set aside.

Lower the heat to medium, add butter and onions with a pinch of salt. Cook for 25-30 minutes, stirring occasionally, until the onions are golden and caramelized.

Add garlic and tomato paste and cook for 1 minute, stirring until fragrant. Pour in the red wine and simmer for 2-3 minutes, scraping up any browned bits from the bottom, until the liquid is reduced by half.

Stir in Worcestershire sauce, broth, thyme, and the bay leaf. Return the roast (and its juices) to the pot. Cover and bake for 2½-3 hours, until the beef is fork-tender.

Remove the roast and shred it into chunks. Discard the bay leaf, then stir the beef back into the onion gravy.

While the roast finishes, bring a large pot of salted water to a boil. Add the potatoes and cook for 15-20 minutes until fork-tender. Drain and mash with butter, half & half, sour cream, salt, pepper, and garlic powder until smooth.

Increase the oven temperature to 400°F. Spread the mashed potatoes over the beef-and-onion mixture in the Dutch oven. Top with shredded Gruyère.

Bake uncovered for 20-25 more minutes, until the cheese is melted, bubbly, and golden brown. Let rest for 10 minutes, sprinkle with chives, and serve warm.