Remove the sausage from the casing. Brown meat and remove from pan. Remove some of the fat if there is too much. Saute the onions in the remaining sausage fat for about 5 minutes. Add potatoes and cook another 5 minutes. Add in the carrots and celery and stir and cook another 5 minutes. Finally add in the garlic and cook another minute. return the cooked meat to the pot and add the stock, oregano, and chile flake. Simmer until the veggies are tender. Meanwhile, toss the brussel sprouts in oil, salt, and pepper, and broil to brown. When the soup is about ready, kill the heat and add in the brussel sprouts. Serve with bread and lots of pecorino romano.