Heat oil in a Dutch Oven over high heat. Sear the beef shank slices for 3-4 minutes per side until well browned. Remove and set aside.
Cut the red onion and spring onions into roughly 1/4-inch thick slices. Cut the potatoes into roughly 1/4-inch thick slices. Cut the red peppers into strips.
In the same Dutch Oven over medium-high heat, cook the pork belly until lightly browned, about 5-7 minutes. Add the prepared onions, spring onions, peppers, and potatoes. Fry for 3-4 minutes, stirring occasionally.
Pour in the beef broth and dark beer. Mince the garlic and crush the chillies, then add to the pot. Place the reserved beef shank slices on top, ensuring they are mostly covered by the liquid. Cover the Dutch Oven with its lid.
Place in a preheated 160°C (320°F) oven for 2 hours 30 minutes. Check the liquid level every 45 minutes and add more broth if needed. The meat should be very tender.
Using a slotted spoon, remove the beef shank slices and about two-thirds of the vegetables and transfer to a serving platter to keep warm.
Mash the remaining vegetables with a generous amount of butter until creamy. Stir well and season with salt and pepper to taste.
Serve the beef shank slices and reserved vegetables on plates, topped with the creamy sauce.
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