Preheat your oven to 425°F (220°C).
In a large mixing bowl, combine the diced red onion, bell pepper, broccoli florets, yellow squash slices, and zucchini slices.
Drizzle the olive oil over the vegetables and sprinkle the Cajun seasoning on top. Toss everything together until well-coated.
Spread the seasoned vegetables in a single layer on a baking sheet. Roast in the preheated oven for about 25-30 minutes, stirring halfway through.
Remove the veggies from the oven and let them cool slightly before serving.