Preheat your oven to 425°F (220°C). This high temperature is essential for achieving that perfect caramelization and crispiness on your veggies.
In a large mixing bowl, combine the diced red onion, bell pepper, broccoli florets, yellow squash slices, and zucchini slices. Ensure that all the vegetables are roughly the same size for even cooking.
Drizzle the olive oil over the vegetables. Sprinkle the Cajun seasoning on top. Toss everything together until the vegetables are well coated in the oil and seasoning.
Spread the seasoned vegetables in a single layer on a baking sheet. Make sure not to overcrowd the pan, as this can lead to steaming rather than roasting.
Place the baking sheet in the preheated oven and roast for about 25 to 30 minutes. Halfway through the cooking time, give the veggies a good toss with a spatula to promote even browning.
The vegetables should be tender and slightly charred when done. If you prefer them a bit more crispy, leave them in for an additional 5 minutes.
Take the roasted veggies out of the oven and allow them to cool slightly before serving. They make a fantastic side dish or can be added to your favorite grain bowl or salad.