Preheat the oven to 160°C
- Season the ribs well with salt and pepper. Heat a large heavy-based ovenproof casserole dish over high heat. Add the oil and cook the ribs in batches until golden brown all over. Remove the ribs and set aside.
- Reduce the heat to medium and add the onion, carrot and celery, stirring occasionally, until golden. Stir in the garlic and mustard and cook for 30 seconds longer. Pour in the beer and stock, stirring to incorporate any caramelised bits from the base of the pot.
- Return the ribs to the casserole along with the rosemary sprigs and bring to the boil.
- Cover with a lid and transfer to the oven. Cook for 3 hours, then uncover and cook for a further 30 minutes.
Serve with buttery mashed potato and steamed green beans.