Place the chicken breasts in a shallow dish. Add 2 tablespoons olive oil and 2 tablespoons peri-peri seasoning, then turn the chicken to coat evenly; set aside to marinate while you prepare the vegetables (or cover and refrigerate if marinating longer).
Remove the chicken from the refrigerator 20–30 minutes before cooking so it comes to room temperature if it was chilled.
Preheat the oven to 450°F (230°C).
Peel the onions and cut into quarters. Cut the sweet potatoes into wedges. If using large corn cobs, cut them in half.
Arrange the sweet potato wedges, quartered onions, and mini corn on a large sheet pan in a single layer with some space between pieces.
Drizzle the vegetables with 2 tablespoons olive oil and sprinkle with 1 tablespoon peri-peri seasoning, then toss lightly to coat.
Roast the vegetables in the preheated oven for 25 minutes, until they begin to brown and are nearly tender.
Remove the sheet pan and make four spaces for the chicken breasts. Place the marinated chicken breasts on the tray among the vegetables.
Return the tray to the oven and roast for 16–18 minutes (or 18–20 minutes for larger breasts) until the chicken is cooked through and the potatoes are tender; internal temperature should reach 165°F (74°C).
Transfer to plates and serve with lemon wedges, fresh cilantro and sliced red chili.
