Preheat the oven to 325 degrees F (160 degrees C). Season short ribs generously with salt and pepper on all sides.

Add vegetable oil to a skillet and set over medium-high heat. Transfer in the short ribs and brown thoroughly on all sides, about 10 minutes. Turn off the heat.

Transfer browned short ribs into a 9x5-inch loaf pan, spacing evenly.

Put the pan used to brown the beef back over medium-high heat, and add onions along with a large pinch of salt. Cook, stirring, until onions are very well browned, 5 to 6 minutes. Add tomato paste and flour; cook and stir for 2 minutes.

Deglaze the pan by adding the sherry wine, and cook for about 1 minute to reduce slightly. Pour in 1 ½ cups beef broth, and stir to combine. Allow the liquids to boil and reduce for a few minutes, or until slightly thickened. Turn off the heat and reserve.

Place the loaf pan on a baking sheet, and distribute carrots and celery evenly over the top. Add thyme and rosemary to the loaf pan, and pour reserved cooking liquids from the pan over the top. Spread everything out evenly, and cover very tightly with heavy duty foil.

Bake in the preheated oven until the meat is tender as tested with a fork, 3 to 3 ½ hours. Turn the oven off.

Carefully remove short ribs to a plate to cool. Remove and discard herbs.

Add remaining ½ cup beef broth to the loaf pan, and stir. Let cool to room temperature, then cover and refrigerate until thoroughly chilled, a few hours or overnight. Wrap and refrigerate the short ribs as well.

To finish, preheat the oven to 325 degrees F (160 degrees C). Remove the loaf pan from the refrigerator, and lift the hardened fat off the top surface, scraping off any sauce or vegetables back into the pan. Transfer short ribs back in, spacing evenly, and baste the tops thoroughly with the cooking liquids.

Bake in the preheated oven until meat is heated through and very tender, 45 minutes to 1 hour. Taste sauce for seasoning and adjust with salt if needed. Serve over mashed potatoes with some additional thyme leaves sprinkled over the top as an optional garnish.