preheat oven: to 325℉
prep the ribs: Pat the short ribs dry and season all over with salt and pepper.
sear the meat: Heat olive oil in a large oven-safe pot or Dutch oven over medium-high heat. Brown the ribs on all sides, then remove and set aside.
cook the aromatics: In the same pot, add the sliced onion and cook until softened, about 5 minutes. Add the garlic and cook 1 more minute until fragrant.
deglaze: Pour in the cognac and whiskey, scraping up any browned bits from the bottom of the pot. Let it reduce slightly, about 2 minutes.
add herbs & braise: Return the short ribs to the pot. Tuck in the thyme and sage. Cover and transfer to oven. Braise for 2½–3 hours, until the meat is fall-apart tender.
make the mashed potatoes: while the ribs slow cook, peel and cube russet potatoes, then boil in salted water until tender.
finish the sauce: Remove the ribs and stir heavy cream into the sauce. Let it simmer on the stove for a few minutes until it’s creamy and silky.
brown butter sage: While the sauce simmers, melt a few tablespoons of butter in a small pan with extra sage leaves until fragrant and lightly browned.
finish potatoes: Drain and return potatoes to the pot. Mash with heavy cream, freshly grated Parmesan, salt, pepper, and the leftover browned butter from the sage. Taste and adjust seasoning.
serve: Spoon the creamy sauce over the short ribs and plate with the mashed potatoes. add the crispy brown butter sage leaves and enjoy!