Preheat your oven to 425°F. Use a large sheet pan (about 18x13 inches) to ensure proper spacing. Toss the halved baby potatoes with 2 tablespoons of olive oil, salt, and black pepper. If your potatoes are larger than bite-size, give them a 10-minute head start in the oven.

In a large bowl, combine 2 tablespoons olive oil, chicken pieces, balsamic vinegar, Dijon mustard, chopped oregano, paprika, chopped garlic, chopped shallot, salt, pepper, and chili flakes to taste. Toss everything together until the chicken is evenly coated.

Nestle the marinated chicken pieces around the potatoes on your sheet pan, making sure everything has space to roast properly. Bake for 25-30 minutes until the chicken reaches 165°F internally and the potatoes are fork-tender. Watch for the chicken to turn opaque and golden at the edges.

While everything roasts, prepare the herbed olive topping by combining the remaining 1/3 cup olive oil, lemon juice, chopped fresh herbs, sliced pepperoncini, torn olives, and sesame seeds in a bowl. In a separate small bowl, mix the Tzatziki sauce with crumbled feta cheese.

Remove the sheet pan from the oven and immediately spoon the herbed olive mixture over the hot chicken and potatoes. Let the pan sit for a few minutes to allow flavors to meld. Serve with the creamy feta Tzatziki sauce on the side.