Heat 1 tablespoon of oil in a large saucepan over medium-high heat. Cook the onion, carrots, and celery for 8 to 10 minutes, until just beginning to color. Add the garlic, thyme, and rosemary, and cook for a further couple of minutes. Remove from the pan and place in the bottom of a slow cooker.
Season the flour generously with salt and pepper. Coat the short ribs well in the flour. Heat the remaining oil in the pan and brown the short ribs in 2 batches until golden, adding any remaining flour with the second batch of ribs. Remove the ribs from the pan and place on top of the vegetables in the slow cooker.
Add the wine to the pan to deglaze, and reduce it by half, scraping the bottom of the pan to release any sediment. Add the stock, tomato sauce, brown sugar, mustard, Worcestershire sauce, and bay leaves, and stir to combine. Pour the sauce over the ribs and vegetables in the slow cooker. Cover and cook on low for 8 hours or high for 4 hours, stirring occasionally. Season to taste. Serve over mashed potatoes or a creamy polenta.