Place currants in a small bowl and pour in enough boiling water to cover. Let plump for 5 to 10 minutes. Drain, reserving the soaking liquid.
Wash and dry spinach leaves and roughly chop.
Heat oil in a large non-stick frying pan over medium-low heat. Add pine nuts and cook, stirring, until light golden, 1 to 2 minutes. Transfer pine nuts to a small bowl and set aside. (Do not wash the pan.)
Add onion and garlic to the frying pan. Cook, stirring, until softened and light golden, 2 to 3 minutes. Add spinach and 2 tablespoons of the reserved soaking liquid. Increase the heat to medium-high and cook, stirring, until spinach has just wilted, about a minute. Stir in currants and pine nuts. Season with vinegar and pepper.