Preheat oven to 400°F and line a large sheet pan with parchment paper.
In a large bowl, toss together bell peppers, onion, zucchini, and sausage pieces with olive oil, Italian seasoning, garlic powder, salt, and red pepper flakes if using.
Spread everything evenly on the sheet pan in a single layer, making sure not to overcrowd.
Roast for 20 minutes, stirring once halfway through.
Add cherry tomatoes to the pan and roast for an additional 5-7 minutes until tomatoes start to blister and sausage is cooked through.
Remove from oven and sprinkle with fresh basil and Parmesan cheese.
Serve over pasta, rice, or with crusty bread to soak up all those delicious juices!