Start by preheating the oven to 300°F and heat dutch oven over high heat.
Generously season short ribs with kosher salt and freshly cracked pepper. Add avocado oil to dutch oven and slowly add short ribs. Sear on all sides until deeply and evenly brown. Remove from dutch oven and place on a plate.
Remove all but 3 tablespoons of fat from dutch oven and add onions, carrots and celery. Cook for 4 minutes, then add garlic and leeks. Season with kosher salt and freshly cracked pepper. Cook for an additional 6 minutes, careful not to brown the vegetables, rather cook vegetables over medium low heat until they are translucent, soft, and fragrant.
Add tomato paste and press into vegetables using the back of spoon or cooking utensil. Cook for two minutes.
Add red wine and cook over medium-heat until slightly reduced, 4-5 minutes.
Add beef broth, thyme, bay leaves and stir. Nestle short ribs back into dutch oven. Cover with lid.
Cook for 2 hours undisturbed. After the 2 hour mark, check short ribs for tenderness. Once meat easily shreds with a fork and meat is falling off the bone, remove from oven. Oven time typically takes 2 hours 45 minutes to 3 hours. Once cooked, discard thyme and bay leaves.
To serve, place short ribs atop Herbed Mashed Potatoes and spoon sauce over. If you prefer a thicker sauce, see below.