Preheat oven to 350 degrees. Heat large cast iron or dutch oven with 3 tbsp olive oil on medium high heat.
Season short ribs with salt and pepper. Place in hot oil and sear each side of the rib, about 8 minutes total. Remove ribs and set aside.
Add 1 tbsp butter, onions, and celery. Saute a few minutes, then add mushrooms and garlic.
Add in 1 tbsp tomato paste and 2 tbsp flour and cook until dark red. Stir red wine and beef broth and Worcestershire sauce. Reduce until half the liquid remains, stirring continually, about 20 minutes. Add carrots, thyme sprigs, and bay leaves, and short ribs to pan.
Cover cast iron tightly with foil and bake for about 2 hours until beef is tender and falls off the bone. Remove from oven rest about 10 minutes. Serve over creamy mashed potatoes.
Keyword beef short ribs, cast iron short ribs, red wine beef short ribs