Sweet-and-Sour Dressing: In a small saucepan over medium heat, combine sugar and vinegar; cook stirring, for 2 minutes, or until the sugar dissolves. Cool. Stir in onion, paprika, mustard, and salt. Transfer mixture to a blender; with motor running, add oil in a slow stream through the feed tube.

Meanwhile, put chicken in a saucepan with 1⁄4 inch water and bring to a boil. Reduce heat to medium-low; cover and simmer, turning once, for 5 minutes, or until just cooked through. Remove chicken, cool and cut crosswise into 1⁄4-inch slices.

Scoop out melon with melon baller or cut into cubes. In a large salad bowl, toss spinach, chicken, avocado, melon, dressing, and salt and pepper to taste.

Tip: The dressing can be made, covered and refrigerated for up to 1 week. The chicken can be cooked, cooled, covered, and refrigerated up to 2 days.