Lightly cooked slices of rhubarb are spooned atop spinach leaves and veiled with a warm sweet-and-sour dressing.
1. Place the rhubarb in a wide saucepan. Sprinkle on the sugar and add enough water to cover by 1 inch. Place over high heat and bring to a boil. Cook, uncovered, for exactly 2 minutes. Remove from the heat and pour through a large sieve into a bowl. Return the liquid to the pan. Stir in the vinegar, salt, and pepper and place over high heat. Cook, uncovered, until the mixture is reduced to ½ cup.
2. Meanwhile, divide the spinach among 4 salad plates. Arrange the rhubarb over the spinach. When the liquid is reduced, remove the pan from the heat and whisk in the oil. Pour over the salads and serve at once.