Preheat oven to 450°F.
Toss potatoes, 2 teaspoons oil and 1/4 teaspoon each salt and pepper in a large bowl; toss to coat. Spread evenly on a rimmed baking sheet. Roast until the potatoes are browned and slightly crisp, 20 to 25 minutes.Turn once halfway through baking.
Whisk 1/2 cup broth and flour in a small bowl; set aside.
Heat the remaining 3 teaspoons oil in a medium saucepan over medium heat. Add mushrooms and onion and cook, stirring often, until soft, 5 to 7 minutes. Add the remaining 1 cup broth; bring to a simmer. Cook until reduced by about half, 8 to 10 minutes.
Add the reserved flour and broth mixture; cook, stirring constantly, until smooth and thickened, 1 to 3 minutes. Stir in chives and the remaining 1/8 teaspoon salt and 1/4 teaspoon pepper; keep warm on low heat. If gravy is too thick, add more broth until you reach desired consistency.
When the potatoes are done, either transfer them to a baking dish or just push them together in the center of the pan and sprinkle with cheese. Return to the oven; bake until the cheese is melted, about 5 minutes more. Serve the potatoes topped with the gravy.