Step 1Spread 1 tablespoon of oil on the bottom of a 13x9-inch roasting pan. Arrange half of the potato slices evenly in the pan. Scatter the onion and garlic over the slices. Season with 1/2 teaspoon salt and pepper to taste. Add the remaining potatoes in an even layer, sprinkle with 1/2 teaspoon salt and more pepper, then drizzle with 1 tablespoon of oil.
Step 2Bake, uncovered, until the potatoes are tender and slightly browned, about 50 minutes.
Step 3Use a fork to lightly beat the eggs in a large bowl with a scant 1/2 teaspoon salt and pepper to taste. Add the warm potato slices, using the back of a spoon to press the slices into the mixture. Set aside at room temperature until most of liquid is absorbed into the potatoes, about 15 minutes.
Step 4Heat 1 tablespoon of oil in a 9-inch nonstick skillet over high heat. Swirl the oil around the bottom and sides of the skillet. When the oil is sizzling hot, add half of the potato mixture, pressing it evenly into place with a spatula. Arrange the spinach leaves evenly over. Add the remaining potato mixture in an even layer. Use a spatula to press the mixture into place. Lower the heat to medium and cook, shaking the pan often to keep it from sticking, until lightly browned, about 4 minutes.
Step 5Slide the tortilla onto a large dish; cover it with a second dish and invert the tortilla. Add the remaining tablespoon of oil to the pan. Heat the oil until sizzling; slide the tortilla into the skillet. Cook over medium heat, shaking the pan often, until lightly browned, about 4 minutes. The tortilla should be cooked through but still moist inside.
Step 6Serve at room temperature, cut into wedges with sharp knife