Preheat the oven to 425°F and prepare a large baking sheet with parchment paper. Combine the potatoes with a drizzle of olive oil, sea salt, and black pepper, then arrange them in an even layer on the sheet. Roast for 20 to 30 minutes until the potatoes turn tender and their edges are beautifully golden, adjusting the time according to their size and freshness. In the meantime, prepare the dressing by whisking together the olive oil, lemon zest, lemon juice, Dijon mustard, minced garlic, rosemary, red pepper flakes, sea salt, and black pepper in a small bowl. Once the potatoes are roasted, drizzle them with the dressing and toss gently, adding as much dressing as desired. Adjust seasoning with additional salt and pepper if needed. Garnish with fresh parsley before serving.