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1
Preheat the oven to 450 degrees F (230 degrees C).
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2
Prick russet potato and sweet potato all over with a fork. Wrap individually in aluminum foil.
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3
Bake in the preheated oven until soft, about 1 hour 15 minutes. Unwrap the potatoes and let cool slightly.
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4
Remove skin from the potatoes and mash flesh in a bowl. Let cool, at least 15 minutes. Add egg and mix with a fork. Mix in all-purpose flour gradually using a spoon; knead together by hand as dough becomes tougher.
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5
Dust your work surface with rice flour. Place the dough on the surface; it will be quite sticky. Cut into 8 pieces; place on a tray and freeze until easier to handle, about 10 minutes.
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6
Take 1 piece of dough from the freezer. Roll out into a long sausage on the dusted work surface. Cut into comparably small gnocchi pieces, the smaller the better. Transfer to a dish covered with plastic wrap and dusted with flour. Repeat with the remaining 7 pieces of dough. Transfer gnocchi to the freezer until ready to cook.
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7
Bring a large pot of lightly salted water to a boil. Cook gnocchi in the boiling water until they float to the top, 3 to 4 minutes.
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8
Combine olive oil, butter, sage, and garlic in a large saucepan over medium-low heat. Cook and stir until butter is melted and flavors are combined, 2 to 3 minutes. Remove garlic and add the gnocchi. Top with Parmesan cheese and mix well.
