Preheat oven to 350ºF. Layer potatoes alternately with onions in a 12-inch oven-proof skillet, sprinkling with salt as you go.
Pour cream overtop and sprinkle with black pepper and nutmeg (if using). Add sage leaves and give the mixture a gentle stir. Transfer to stove and bring to a rapid simmer over medium-high heat.
Reduce heat to low and cook uncovered for 15 minutes, gently stirring occasionally, until cream is slightly thickened and potatoes are about half cooked. The cream will taste salty as it simmers but the salt will move into the potatoes to perfectly season them.
Dot the top of the potatoes with butter, sprinkle with parmigiano reggiano, and bake until cheese is golden and potatoes are tender, about 15 minutes.
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Directions
Preheat oven to 350ºF. Layer potatoes alternately with onions in a 12-inch oven-proof skillet, sprinkling with salt as you go.
Pour cream overtop and sprinkle with black pepper and nutmeg (if using). Add sage leaves and give the mixture a gentle stir. Transfer to stove and bring to a rapid simmer over medium-high heat.
Reduce heat to low and cook uncovered for 15 minutes, gently stirring occasionally, until cream is slightly thickened and potatoes are about half cooked. The cream will taste salty as it simmers but the salt will move into the potatoes to perfectly season them.
Dot the top of the potatoes with butter, sprinkle with parmigiano reggiano, and bake until cheese is golden and potatoes are tender, about 15 minutes.