In a large pot, heat the oil over medium heat. Once it's hot, add the sausage. Cook until browned nicely, then remove it to a towel lined plate and cover with foil. Place the butter into the pot. Once it melts, add the onion, carrot and celery with a big pinch of salt. Cook, stirring often, until softened, about 6-8 minutes. Then stir in the garlic and cook another couple of minutes. Increase the heat to med-high.