In a large Dutch oven, melt butter over medium-high heat until foaming subsides (do not allow butter to brown), about 2 minutes. Add sausage and mash with a stiff whisk or potato masher to break up into fine pieces (largest pieces should be no bigger than 1/4 inch). Cook, stirring frequently, until only a few bits of pink remain, about 8 minutes. Add onion, celery, garlic, and sage and cook, stirring frequently, until vegetables are softened, about 10 minutes. Remove from heat and add half of chicken stock.