Preheat oven to 375°F. Line a large baking sheet with parchment paper and set aside. Unwrap pie shells and set aside at room temperature until pliable and no longer cold, about 30 minutes.
Meanwhile, heat 1 1/2 tablespoons oil in a large skillet over medium-high heat. Add onion, salt and pepper and cook, stirring occasionally, until golden brown, 6 to 8 minutes. Add potatoes and 1/4 cup water, cover and simmer, stirring occasionally and scraping up any browned bits, until tender, 8 to 10 minutes. Transfer to a large bowl and briefly mash until chunky. Set aside to let cool. Reserve 1/4 cup of the cheese, then add remaining to bowl with potato mixture along with olives, cilantro, salt and pepper. Gently fold together and set aside.
Invert one of the pie shells onto a floured surface, removing the tin, then gently flatten and roll out into a 12-inch circle. Using a small bowl as a cookie cutter, cut four 5-inch circles out of the dough, reworking the scraps as needed. Transfer circles to a large plate and cover with a damp paper towel then repeat process with second crust to make 8 circles total.
Working with one circle of dough at a time, brush edges with water then arrange about 1/4 cup of the potato filling in the middle. Fold over to make a half moon. Pressing out air as you go, pinch edges together snugly, crimping them, if desired. Transfer to prepared baking sheet. Brush empanadas with remaining 1/2 tablespoon oil and sprinkle tops with reserved cheese. Bake until golden brown and crisp, about 45 minutes.
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