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