To prepare the dressing: Whisk water, vinegar, sugar, olive oil, cornstarch, salt, onion powder, mustard powder, and poppy seeds in a saucepan. Bring mixture to boil; cook, whisking constantly, until dressing is thickened. Remove from heat. Pour into a container and set in refrigerator until chilled, approximately 1 hour.
To prepare the peppers: Bring a large pot of salted water to a boil. Fill a large bowl with ice and 1 c. of water to make an ice bath. Cut the tops off the peppers, and remove the seeds. Cook peppers in boiling water for 5 minutes; drain. Rinse in cold water, then set into ice bath. Turn the peppers every minute to throughly chill. Once the peppers are completely chilled, use a paring knife to cut faces into the peppers, much like carving a jack-o-lantern.
For the salad: Sprinkle salt inside the peppers. Toss salad greens in poppy seed dressing. Fill peppers with salad mix and top with a sprinkle of salt and pepper.