Preheat the oven to 150°C (fan). Use a large, heavy-based ovenproof pot or casserole dish with a lid.
Season the short ribs generously with salt and pepper. Heat the oil in the pan over medium-high heat and brown the ribs in batches until nicely coloured on all sides. This should take about 8–10 minutes. Set the ribs aside on a plate.
Lower the heat to medium. In the same pot, add the sliced onions, crushed garlic, and diced celery. Cook gently for 10 minutes until softened and golden.
Stir in the tomato purée and cook for 1 minute, stirring to coat the vegetables.
Pour in the ale, scraping the bottom of the pan with a wooden spoon to release any caramelised bits. Let it bubble for a minute or two.
Add the bay leaves, malt vinegar, and black treacle, stirring well. Return the browned ribs to the pot, nestling them into the liquid and veg.
Cover the pot tightly with a lid or foil. Transfer to the oven and cook for 2.5 hours.
After 2.5 hours, remove the lid. Spoon off any fat from the surface. Baste the ribs with the cooking liquid. Return to the oven uncovered for a final 30 minutes, basting again halfway through.
Check the meat – it should be tender and easily pull away from the bone. Transfer the ribs to a warm plate.
Place the pot back on the hob over medium heat. Simmer the sauce for 5–10 minutes to thicken slightly if needed. Taste and adjust seasoning.
Serve the ribs hot, spooning over some of the thickened glaze and scattering with chopped parsley.