Warm butter and oil in a large pot over medium heat. Add onions and cook, stirring occasionally, until golden brown (15 to 20 minutes). Reduce heat, add garlic, sage and sugar and continue to cook on medium-low heat, stirring, until garlic softens (5 to 8 minutes). Add port. Increase heat to high and cook for 3 minutes, stirring constantly to deglaze pan, scraping up the flavorful bits. Add chicken stock. Bring to a boil, reduce heat, cover and simmer 10 more minutes. Season to taste with salt and freshly ground black pepper.