While potatoes roast, combine remaining 1 Tbsp. oil, shallots, garlic, and sage in a medium skillet over medium-heat; cook 2 minutes. Whisk together broth, vinegar, Dijon mustard, and maple syrup; add to pan, and whisk to combine. Bring to a boil; reduce heat and simmer 10 to 12 minutes, or until mixture begins to reduce. Stir in butter, and remaining 1/4 tsp. each of salt and black pepper. Remove from heat.