Preheat oven to 325°F. Season the short ribs all over with the kosher salt and pepper. Set aside.
Heat oil in a Dutch oven or large heavy pot over medium heat. Brown the short ribs on all over, about 3 minutes each side. Transfer to a platter.
Add the onion, celery, carrots, garlic, tomato paste, dried oregano, ground coriander, ground cumin and red pepper flakes to the pot. Stir often over medium high heat, until vegetables are softened, about 10 minutes.
Add the wide strips of orange zest (I use a vegetable peeler), orange juice and water to pot, scraping up any browned bits and season with kosher salt and pepper.
Add ribs with any juices from the platter, pressing them down into the vegetables until they’re submerged. Cover pot and braise ribs in the oven for 2 hours.
Carefully remove vegetables and short ribs to a platter. Cover with aluminum foil to keep warm.
Bring the braising liquid to a boil on the stovetop, then stir in the cornstarch mixed with some water, lower heat and stir until thickened.
Place the short ribs on mashed potatoes and pour the orange sauce over the short ribs. Top with cilantro leaves, remaining teaspoon of orange zest serve with braised vegetables.