Pour the olive oil into a medium skillet and add the garlic cloves. Put the pan over medium heat. Cook, stirring occasionally, until the garlic is fragrant and lightly toasted, about 3-4 minutes. Discard the garlic. Add the onion, and season with salt and pepper. Cook, stirring occasionally, until the onion is golden and tender, about 3-4 minutes. Stir in the currants and cinnamon and cook 30-60 seconds longer. Add the drained spinach, and season with salt and pepper. Toss well to coat, breaking up the spinach. Remove the pan from the heat and drizzle in the sherry vinegar. Transfer the mixture to a large bowl and let cool slightly. Do Ahead: The spinach mixture can be refrigerated for up to 1 day.