Preheat the oven to 160 °C. Roast the whole, skin-on red onions in a baking dish for 40 minutes until soft and caramelized. Cool slightly.
Cut off the root end, peel off the papery skin and tough outer layer (with membrane) so the glaze can adhere. Using a spoon handle, gently remove the inner layers to leave hollow onion cups. Reserve the extracted centers for the filling.
Lower the oven to 140 °C. Lightly dust the diced short rib with flour. Sear in a hot skillet until browned outside but still raw inside; remove.
Lower the heat, melt the butter in the same pan, and sauté the diced onion and carrot until softened. Return the beef, add the port and enough stock to cover.
Place a parchment cartouche on the liquid, cover with a lid, and braise at 140 °C for 4 hours until the beef shreds easily.
Strain the braising liquid into a saucepan for the glaze. Mince the reserved roasted onion centers into a fine paste.
Shred the short rib. Fold in the onion paste, mustard, parsley, and a splash of braising stock. Season with salt and pepper. Transfer to a piping bag.
Pipe the filling into the hollowed onions. Simmer the remaining braising stock into a glossy glaze, skimming fat as needed. Spoon over the onions until coated and serve hot.
Notes
Prep about 30 minutes; cook about 4 hours 40 minutes. Yield: 4 stuffed onions.
Inspired by Fallow; based on https://www.youtube.com/shorts/i0eRWJIFqPc