In Spain, "tortilla" refers to an omelet.
Heat oven to 400 degrees with rack in center. In a medium saucepan over high heat, cover potatoes with 4 cups water and salt. Bring to a boil. Reduce to a simmer, and cook until tender, about 5 minutes. Drain, and set aside.
Slightly beat eggs with salt and pepper. Set aside. Heat oil in a 10-inch ovenproof nonstick saute pan over medium-high heat, until just beginning to smoke. Add potatoes, and cook, stirring occasionally, until evenly golden brown, 10 to 12 minutes. Add onions, and continue to cook until wilted and beginning to brown, 2 to 3 minutes more. Add eggs, sprinkle with chives, and swirl pan to distribute evenly. Transfer to oven.
Bake until center is set and sides are puffy and golden, about 10 minutes. Reduce oven temperature if eggs brown too quickly. Do not overcook, or eggs will get tough. Let cool slightly before slicing into small wedges.