Preheat the oven to 425°F (use convection if available). Line a rimmed sheet pan with heavy-duty aluminum foil if desired and lightly spray or oil it; set aside.
Place the halved or quartered potatoes and diced bell pepper on the sheet pan. Drizzle with 3 tablespoons olive oil, then sprinkle with 1 teaspoon kosher salt, 1/2 teaspoon black pepper, and 1 to 1½ teaspoons Madras curry powder. Toss with your hands or a spatula to coat evenly and spread in a single layer.
Bake for about 15 minutes, until the potatoes are partially cooked and beginning to brown.
Remove the pan from the oven. Scatter the diced chicken breast evenly among the vegetables. Drizzle with the remaining 3 tablespoons olive oil and sprinkle with the remaining 1 teaspoon kosher salt, 1/2 teaspoon black pepper, and the remaining curry powder. Toss gently to combine and redistribute in a single layer.
Return the pan to the oven and bake for an additional 15 minutes, or until the chicken is cooked through (no longer pink) and the potatoes are fork-tender.
Remove the pan, add the frozen peas directly to the hot pan, and return to the oven for 1–2 minutes just to warm them through. Serve immediately.
