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1
Peel the potatoes and place potatoes in a large pot, cover with cold water, and bring to a boil. Reduce to a low boils and cook until tender, about 30 minutes. Drain potatoes, and set aside until cool. Refrigerate until chilled, at least 1 hour. Grate potatoes using the large holes on a cheese grater.
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2
Meanwhile, grate the zucchini into a sieve using the large holes of a cheese grater. Sprinkle lightly with a little salt and let sit for about 15 minutes to draw out the moisture. Gather the grated zucchini into a cheesecloth and wring out as much moisture as possible. (Alternately you can squeeze out the moisture with your hands, but it's a little more difficult).
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3
Heat butter and the oil in an 8? nonstick skillet over medium-low heat. When the butter has melted, add potatoes and zucchini, sprinkle with salt, and mix well, coating potatoes with fat. Using a spatula, gently press potatoes, molding them to fit the skillet. Cook, shaking skillet occasionally, until edges are golden brown, about 20-30 minutes.
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4
Cover skillet with a large inverted plate, invert the rosti over onto plate, then slide it back into the skillet, cooked side up; cook until golden brown on the bottom, about 20 minutes. Transfer to a cutting board, sprinkle with salt, and cut into wedges to serve.
