Preheat your oven to 250 degrees.
Pull ribs out of the fridge about 20 minutes before cooking. You don't want to put ice cold ribs on a hot pan because it could lead to an uneven browning of the meat. Season the ribs with salt and pepper on all sides.
In a bowl, mix remaining ingredients (except for the butter and oil).
Heat a large dutch oven or braiser over high heat on the stove. Once very hot, add the butter and oil.
Carefully sear the short ribs on each side for about 30 seconds. Once browned on all sides, remove ribs from pan and transfer to a plate.
Pour sauce into the same pan you seared the ribs in. Bring to a simmer for about 4-5 minutes. Transfer ribs back to the pan with sauce and cover with lid or foil.
Transfer ribs to the oven for 2 1/2 hours. When done, ribs will be very tender. Remove from oven and allow to cool slightly.
To thicken the sauce, carefully remove ribs from dish and bring sauce to a simmer on your stovetop. Simmer, stirring occasionally until sauce thickens to your liking.
Serve short ribs over mashed potatoes with sauce and a sprinkle of fresh chopped green onions (optional).