Step 1

In a large bowl, whisk together the vinegar, soy sauce, mustard, rosemary, garlic and 3 tablespoons of oil. Season to taste with salt and pepper. Pour half of the marinade into a separate bowl and set aside. Cut each onion through the root into 6 to 8 wedges and add to one of the bowls along with the chicken. Toss well to evenly coat the chicken and onions with the marinade and set aside at room temperature for 30 minutes. If looking to get ahead, the chicken can marinate for up to 5 hours covered in the fridge.

Step 2

Heat your oven to 400 degrees F and drizzle the bottom of a small roasting pan or a sheet pan with a little olive oil. Remove the chicken and onions from the marinade and arrange on the prepared pan, making sure to leave space between each piece of chicken. Discard this bowl of marinade. Transfer to the oven to roast for 25 minutes.

Step 3

Meanwhile, cut the radicchio into 8 wedges, add to the other bowl of marinade and toss to coat. After the chicken has roasted for 25 minutes, add the radicchio onto the pan along with any of its marinade and crumble the feta into large chunks overtop. Return to the oven until the chicken is cooked, the vegetables are tender and the cheese is lightly golden brown, 15 to 20 minutes.