1. Heat water, milk, salt, black pepper and sage to a boil, over medium high heat, stirring occasionally. Transfer to the bowl of an electric mixer fitted with the whip attachment.
2. Pour mashed potatoes in a steady stream while whipping at medium speed, until potatoes begin to thicken, about 1 min., scraping down sides of the bowl once during mixing.
3. Add brown butter, slowly in a steady stream and whip until fully incorporated, about 1 min. Increase speed to high and whip until light, about 3 min., scraping down sides of the bowl once during mixing. Transfer to steamtable and serve.