Begin by preparing your vegetables: thinly slice the cucumber, onions, and both bell peppers, and quarter the tomatoes.
For Salad:
Combine all the prepared vegetables in a large mixing bowl and toss them together until evenly distributed.
For Dressing:
In a saucepan, mix cider vinegar, celery salt, mustard seed, granulated sugar, red pepper, black pepper, salt, and water. Heat this mixture until it reaches a boil and let it bubble for one minute before removing from heat.
Pour the hot dressing over the mixed vegetables in the bowl.
Cover the mixture and refrigerate for several hours or overnight to enhance the flavors.