Heat olive oil in a medium skillet over medium-high heat. Add mushrooms; sauté 6 minutes or until tender and browned. Stir in 1 Tbsp. butter and minced shallot; sauté 2 minutes. Add salt and ground black pepper to taste.
Melt 1/2 cup 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. Add mushrooms. Cook over medium heat, stirring occasionally, 5 minutes.
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.