Generously season both sides of roast with salt and pepper. Sprinkle flour over the top until well coated then pat it into the meat. Shake off any excess. Heat vegetable oil in a large frying pan over medium-high heat until hot. Sear the roast on both sides for 5-6 minutes each, until well browned. Remove from the frypan and set aside. Reduce the heat to medium then stir in mushrooms and butter; cook for 3-4 minutes.
Stir in onion; cook for 5 minutes, until onions are translucent and begin to brown. Add garlic, stir for about a minute. Stir in 1 1/2 tablespoons flour then cook and stir for about 1 minute. Add tomato paste then cook for another minute.
Slowly add chicken stock, stir to combine then return to a simmer. Remove frypan from the heat. Place carrots and celery in the slow cooker. Place roast over the vegetables then pour in any accumulated juices. Add rosemary and thyme.
Pour onion and mushroom mixture over the top of the roast. Cover slow cooker, turn to high and cook the roast for 5-6 hours, until the meat is fork tender. Skim off any fat from the surface then remove bones. Season with salt and pepper to taste.