Prepare the dressing by combining oil from sun-dried tomatoes, white wine vinegar, minced garlic, Dijon mustard, white sugar, salt, and pepper in a mixing bowl.

Cook spiral pasta in salted water for about 10-12 minutes until al dente, then drain and rinse under cold water.

Combine cooled pasta with baby spinach, shredded cooked chicken, halved cherry tomatoes, and sliced red onion in a large mixing bowl.

Pour the prepared dressing over the salad mixture and toss gently for 1-2 minutes until evenly coated.

Transfer the salad to a serving bowl, let it rest for 10 minutes at room temperature, and then sprinkle with crumbled feta cheese.

For make-ahead, reserve some pasta cooking water to mix with the dressing right before serving.