First, preheat your oven to 400°F (200°C). While the oven is heating up, wash and chop your broccoli, cauliflower, and carrots. Cut the boneless, skinless chicken breasts into bite-sized pieces, aiming for about 1-inch cubes so they cook at the same rate as the vegetables.
Place the prepared chicken and vegetables into a large mixing bowl. Drizzle them with olive oil and sprinkle with black pepper. Use your hands or a large spoon to toss everything together until the chicken and vegetables are evenly coated.
Spread the coated chicken and vegetables in a single layer on a large baking sheet. It’s important not to overcrowd the pan, as this will cause the ingredients to steam instead of roast. If needed, use two baking sheets to give everything enough space to get nicely browned and tender.
Place the baking sheet in the preheated oven and bake for 20-25 minutes. During this time, the chicken should cook through and the vegetables should become tender-crisp. You can check the chicken for doneness by cutting into a piece; it should no longer be pink inside.
Remove the baking sheet from the oven. Pour the sweet chili sauce evenly over the chicken and vegetables. Gently toss everything on the pan to ensure it's all coated in that delicious sauce.
Return the pan to the oven and bake for another 5-10 minutes. This final bake allows the sauce to heat through, thicken slightly, and caramelize on the chicken and vegetables. Once it's bubbly and fragrant, your meal is ready to be served.