1. Arrange the spinach leaves on a platter. Place the rhubarb in a skillet with enough water to cover by 1 inch; add the sugar. Bring the rhubarb to a gentle boil over medium-low heat and simmer until the sugar has dissolved and the rhubarb is lightly cooked, about 2 minutes. Remove the rhubarb with a slotted spoon and distribute over the spinach.
2. Stir the vinegar into the liquid left in the skillet, raise heat to medium, and bring to a boil. Return heat to medium-low and cook until the liquid has reduced to about 3/4 cup, about 10 minutes. Remove from the heat, whisk in the olive oil, and pour the hot dressing over the spinach and rhubarb to wilt the lettuce. Divide salad between 2 plates; serve warm.