In a medium bowl, combine the zucchini and red onion. Set aside.
In a small saucepan, mix together the vinegar, white sugar, brown sugar, salt, pepper, mustard, and celery seed. Bring to a boil over medium heat, stirring constantly.
Once the mixture reaches a boil, reduce the heat to low and simmer for 5 minutes.
Add the zucchini and onion mixture to the saucepan and simmer for an additional 10 minutes.
Remove from heat and let cool. Serve at room temperature.
Interesting Facts
Zucchini relish is a popular condiment in the United States and Canada.
It is often served as a topping on hot dogs and hamburgers.
It can also be used as a substitute for pickles in recipes.