Preheat oven to 325°F (160°C).
Heat vegetable oil in a large oven-safe pot over medium-high heat.
Season beef chuck roast generously with salt and black pepper.
Sear the roast on all sides until browned, about 4 minutes per side. Remove and set aside.
Add sliced onion and minced garlic to the pot and sauté until softened, about 3 minutes.
Stir in tomato paste and cook for 1 minute.
Pour in red wine and beef broth, scraping up browned bits from the bottom of the pot.
Return the roast to the pot and add carrots, celery, thyme, and rosemary.
Cover the pot and transfer to the oven. Slow braise for about 3 hours, or until the meat is tender and pulls apart easily with a fork.
Remove roast and vegetables to a serving dish, cover to keep warm.
Strain the cooking liquid into a saucepan and bring to a simmer.
Whisk together flour with a small amount of water to make a slurry.
Gradually whisk the slurry into the simmering liquid to thicken the gravy.
Season gravy with salt and pepper to taste.
Slice the pot roast and arrange on a shallow white plate. Surround with carrots and celery.
Pour the rich brown gravy generously over the meat and vegetables.
Garnish with fresh thyme or rosemary if desired and serve warm.