Melt butter in a Dutch oven over medium heat; whisk in flour, and cook, whisking constantly, 3 to 4 minutes or until mixture is light brown and smooth. Slowly whisk in 4 cups stock. Increase heat to high; bring to a boil, whisking occasionally. Reduce heat to medium; stir in sage and next 4 ingredients. Simmer, stirring occasionally, 10 to 15 minutes or until desired thickness.
To make ahead, cool gravy completely. Cover and chill up to 3 days. Cook gravy and, if desired, 5 to 6 Tbsp. stock in a Dutch oven over medium-low heat, stirring occasionally, 15 to 20 minutes or until hot.
Mushroom Gravy: Heat 1 1/2 Tbsp. olive oil in a medium skillet over medium-high heat. Add 1 (8-oz.) package sliced cremini mushrooms; sauté 6 minutes or until tender and browned. Stir in 1 Tbsp. butter and 1 Tbsp. minced shallot; sauté 2 minutes. Add salt and ground black pepper to taste. Prepare recipe as directed through Step 1 or Add mushrooms. Cook over medium heat, stirring occasionally, 5 minutes. Hands-on 35 min., Total 45 min.
Fresh Herb Gravy: Prepare recipe as directed through Step 1 or Stir in 1 Tbsp. chopped fresh flat-leaf parsley, 1 tsp. thinly sliced fresh chives, 1/2 tsp. chopped fresh tarragon, and 1/2 tsp. fresh lemon juice. Cook over medium heat, stirring occasionally, 5 minutes. Hands-on 25 min., Total 35 min.
Sherry Gravy: Prepare recipe as directed through Step 1 or Remove pan from heat; stir in 1/4 cup sherry, bourbon, or brandy. Return to medium heat. Cook, stirring occasionally, 5 minutes. Hands-on 25 min., Total 35 min.