Preheat the oven to 450 degrees.

Place the potatoes and onions in a single layer on two cookie sheets. Drizzle with olive oil and sprinkle with salt and pepper.

Roast for 10 minutes. Remove from oven, flip the potatoes, add the chopped garlic, and cook for 6-8 additional minutes or until the potatoes begin browning. Remove from oven and allow to cool slightly.

Meanwhile, combine the Dijon mustard, lemon juice, red wine vinegar, and sherry. Whisk to combine. While whisking, slowly add the olive oil until combined. Stir in the parsley and rosemary. Taste, then add salt and pepper.

If making the deconstructed salad, layer the potatoes in red, white, blue pattern on plates. Thinly slice the roasted onions and place a few on top of potatoes. Drizzle with the dressing.

To make the mixed potato salad, place the potatoes in a large bowl. Thinly slice the roasted onions and add them to the potatoes. Pour the dressing over top and toss to coat.