Preheat over to 425ºF. To make the dressing, add 1/4 C olive oil, vinegar, mustard, garlic, herbs, and salt and pepper into a mason jar (or container of choice). Shake to emulsify and set the dressing aside.
Next, toss cauliflower florets in a large bowl with some olive oil to coat, salt, and pepper. Spread out evenly on a baking sheet and cook until they start to crisp up and brown, about 30 minutes. Remove from the oven and pour into a large mixing bowl. Add the parsley leaves and drizzle with the rosemary thyme vinaigrette to coat. Taste. Season with salt and pepper if necessary. Eat warm or at room temperature.