Preheat your oven to 350°F (175°C). Pat the beef dry with paper towels and generously season all sides with salt and black pepper.

Heat the oil in a large oven-safe pot or Dutch oven over medium-high heat. In batches, sear the beef pieces until they are deeply browned on all sides, around 3–4 minutes per side. Once done, transfer to a plate and set aside.

Reduce the heat to medium. Add the onion and leeks to the pot, cooking and stirring often until they are softened and lightly golden, about 4–5 minutes. Toss in the garlic and carrots, letting them cook for about a minute until fragrant.

Stir in the tomato paste, cooking for another minute or two until it darkens a bit and clings to the vegetables.

Pour in the beef broth and water, using a wooden spoon to scrape any browned bits from the bottom of the pot.

Return the beef and any accumulated juices back into the pot. Add the thyme sprigs and bay leaf—make sure the liquid covers about halfway up the meat. Cover the pot and transfer it to the oven.

Cook for 2 hours and 15 minutes, then remove the lid and continue cooking for another 30–40 minutes, until the beef is fork-tender and the sauce has slightly thickened.

Remove the pot from the oven. Skim off any excess fat from the surface, cover, and let it rest for 30–45 minutes. Before serving, reheat gently on the stovetop.

Serve by spooning mashed potatoes onto plates, topping with the braised beef and vegetables, and ladling the savory pan juices over. Finish with a sprinkle of fresh chives.