Preheat the oven to 425°F (218°C).
Prepare the vegetables: remove stems and seeds from the bell peppers and chop into 1-inch cubes; thinly slice the small yellow onion.
On a large sheet pan (or two smaller pans), toss the chopped peppers, sliced onion, 2 tablespoons olive oil, 1/2 teaspoon salt, 1/4 teaspoon pepper, 1/2 teaspoon garlic powder, and 1 teaspoon dried basil until evenly coated. Spread into a single layer so pieces are not crowded.
Roast the vegetables for 10 minutes, then remove the pan and add the cherry tomatoes and 1/2 tablespoon minced garlic on top; toss gently to combine and spread in an even layer again.
Return the pan to the oven and roast for another 10 minutes, until vegetables are tender and lightly browned. If vegetables are crowded they will steam instead of roast, so give them space.
Meanwhile, bring a large pot of water to a boil and generously salt it. Cook the ravioli according to package directions, subtracting 1 minute from the stated time, then drain.
When the vegetables are done, remove the sheet pan from the oven and immediately add the drained ravioli on top of the roasted vegetables. Spoon the 1/3 cup basil pesto evenly over the ravioli and vegetables and toss on the sheet pan to combine.
Return the pan to the oven for 1–2 minutes to warm everything together, then remove and serve right away. Drizzle with extra olive oil if desired and top with freshly grated Parmesan and fresh basil or parsley.