Combine onion, vinegars, sesame seeds and paprika in a small bowl. Mix in 2 Tbsp. sugar. Gradually whisk in olive oil. Season with salt and pepper.
Melt butter in a heavy skillet over medium heat. Add almonds and stir until they begin to color about 2 minutes. Sprinkle remaining 1 Tbsp. sugar over the almonds and stir until they turn golden, about 2 minutes longer. Transfer almonds to a bowl and cool. Break up almonds as they cool if they begin sticking together. (Dressing and almonds may be prepared several hours ahead. Cover separately and let stand at room temperature.)
Combine spinach and apples in a large bowl. Toss with dressing. Mix in almonds.