Preheat the oven to 425 degrees. Line a baking sheet with tin foil and coat with cooking spray.
Roast the potatoes by cutting them in half and placing them on the baking sheet. Drizzle olive oil on the potatoes and season with sea salt and freshly cracked pepper, to taste. Toss to coat evenly, then place in the oven and roast for 15-20 minutes, or until they are tender.
Meanwhile, make the vinaigrette by combining the olive oil, lemon juice, red wine vinegar, Dijon mustard, minced garlic, oregano, sea salt, and freshly cracked pepper to taste in a small jar. Seal with a lid, shake well, and set aside.
Remove the potatoes from the oven and toss with half of the vinaigrette. Set aside until they cool.
To finish, once the potatoes have cooled completely, add the grape tomatoes, red onion, kalamata olives, parsley, and feta cheese.
Drizzle the remaining vinaigrette on top and toss to coat evenly.