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