Preheat the oven to 400 F.
While oven is heating, toss the onion, celery, carrot and garlic with 1 tbsp of canola oil and place on a baking tray. After oven has heated, roast in the oven for 30 minutes. Stir the vegetables halfway through roasting.
While vegetables are roasting, take the short ribs from the refrigerator and remove the packaging. Pat dry with a paper towel and begin trimming the top of the short ribs to remove the silver skin and any large amounts of fat on top.
Heat a large cast iron pan over medium high heat.
Add 2 tbsp of oil to the pan. Season the short ribs with salt and pepper all over.
Begin to sear the short ribs on all sides in the cast iron pan. You will have to do this in at least two batches depending on the size of your pan. Set aside all the seared short ribs in a large Dutch oven.
Once the vegetables are finished roasting, toss with the tomato paste. Then put vegetables in the Dutch oven with the short ribs. Add in the rosemary, thyme, peppercorns, BBQ sauce and beef stock.
Cover the Dutch oven with a lid and bring to a simmer. Once it is simmering, turn down the heat to low and simmer for 3 hours.
Turn the heat off and let the short ribs sit for 30 minutes.
Remove the short ribs from the pot and set aside.
Strain the liquid and return it back to the pot. Whisk in the instant flour and bring back to a simmer. Simmer for 3-4 minutes. Serve alongside the short ribs.
