Warm the olive oil in a large, heavy skillet over medium heat. Add the pancetta and cook until lightly golden, about 5 minutes. Remove the pancetta from the pan with a slotted spoon and transfer to a large bowl. Add the spinach, ricotta cheese, asiago cheese, pepper, and nutmeg. Stir to combine. Stuff the shells with about 2 tablespoons of the spinach mixture each and place the stuffed shells in a large, buttered baking dish
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