Preheat the oven to 375°F.
Spread potatoes out in a single layer on a large rimmed baking sheet.
Roast until barely golden, about 20 minutes.
Stir potatoes and continue roasting until tender, about 10 minutes more, adding water to the pan if needed to prevent sticking.
Meanwhile, heat a large skillet over high heat.
Cook onion and mushrooms, stirring occasionally, until golden brown, about 10 minutes.
Add potatoes, garlic and sage, stirring to scrape up any bits of onion and mushroom.
Cook until potatoes just begin to brown on the edges, about 10 minutes, adding a small amount of water if needed to prevent burning. Garnish with chives.
If not serving immediately, let cool, then store in an airtight container in the refrigerator for up to 1 day.
Reheat in a 425°F oven for 10 minutes just before serving.