Preheat the oven to 425 degrees F. Spread the cauliflower florets and onions on a baking sheet. Spray or drizzle with olive oil. Season all over with salt, pepper and the garlic powder.
Roast the cauliflower for 20 to 25 minutes, until golden.
Place the potatoes in a pot of salted cold water and bring it to a boil. Boil for 15 minutes, or until fork tender. Drain the potatoes and let them cool. Once cool, slice the potatoes in half.
In a large bowl, combine the potatoes and cauliflower. Add the celery.
In a small bowl, whisk together the mayonnaise, dijon, pickles, pickle juice and vinegar. Stir in the herbs. Add a pinch of salt and pepper and stir.
Pour the dressing over the potato salad. Toss well until everything is coated. Taste and season more if needed.
Serve immediately! You can also place this in the fridge for 30 to 60 minutes before serving. YUM.