Preheat your oven to 425°F (220°C).
Wash the baby potatoes thoroughly and cut them in half if they are larger than a golf ball.
In a mixing bowl, toss the potatoes with olive oil, salt, black pepper, chopped rosemary, and thyme until evenly coated.
Spread the seasoned potatoes out on a baking sheet in a single layer.
Roast the potatoes in the preheated oven for 25-30 minutes, or until they are crispy on the outside and tender inside, tossing halfway through for even browning.
While the potatoes are roasting, prepare the dressing by whisking together apple cider vinegar, Dijon mustard, and chopped parsley in a mixing bowl.
Once the potatoes are finished roasting, remove them from the oven and let them cool slightly.
If using, add the thinly sliced red onion to the roasted potatoes.
Pour the dressing over the warm potatoes and toss gently to combine.
Serve warm or let cool to room temperature, then refrigerate for at least 30 minutes before serving.