Preheat the oven to 350 degrees.
Heat a large dutch oven over medium-high heat. While the dutch oven is heating, liberally season the short ribs with salt and pepper (I use about 1 tbsp of salt).
Once the dutch oven is hot, add the olive oil. Then, place half the short ribs in the pot. Sear the short ribs for 4-5 minutes per side until they have developed a deep brown crust. Remove the short ribs from the dutch oven and repeat this process with the other half of the short ribs.
Once all of the short ribs have been seared and removed from the pan, turn down the heat to medium low.
Add the sliced onion, diced celery, diced carrot, smashed garlic, and fennel to the dutch oven. Cook the veggies for 4-6 minutes and then stir in the mustard and tomato paste until well combined and the tomato paste turns a deep color.
Pour in the beef broth and red wine. Then, add the thyme and dried bay leaves to the pot. Return the short ribs to the pot, nestling them in the liquid. Place the lid on the dutch oven and then place the short ribs in the oven. Braise the short ribs at 350 degrees for about 3 hours or until they are fork tender. When you have about 45 minutes left, add the remaining carrots to the dutch oven.
When the short ribs are done and fork tender, remove the short ribs from the dutch oven and use a spoon to skim off any excess fat from the top of the braising liquid. Heat the dutch oven over medium heat to bring the braising liquid to a simmer. Mix the 2 tablespoons of cornstarch with about ¼ cup of cool water until the cornstarch is dissolved to make a slurry. Add the slurry to the pot. The braising liquid will thicken into a gravy. Return the short ribs to the pot, nestling them in the gravy and ensure that everything is heated through before turning off the heat.
Serve the short ribs and gravy over mashed potatoes or polenta and enjoy!