Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper.
Crush the cornflakes in a zip-lock bag until you have a fine breadcrumb-like texture. Set aside.
Pat the chicken breast dry and lightly season with salt and pepper if desired.
Dredge the chicken breast in the crushed cornflakes, pressing gently to ensure it sticks to the surface.
Place the cornflake-coated chicken on the prepared baking sheet.
In a small bowl, prepare the sweet and sour sauce by mixing pineapple juice, soy sauce, white vinegar, and cornstarch until smooth.
Brush half of the sauce over the chicken to add flavor and help develop a crispy glaze during baking.
Arrange the chopped broccoli and sliced bell pepper around the chicken on the baking sheet. Drizzle the olive oil over the vegetables and toss to coat evenly.
Place the baking sheet in the preheated oven and bake for 20-25 minutes, or until the chicken is cooked through and the vegetables are tender and slightly charred at the edges.
Halfway through baking, brush the remaining sauce on the chicken to enhance the sweet and sour flavor.
Once done, remove from oven, let cool slightly, and serve hot.