Preheat your oven to 400°F (200°C). This will ensure your vegetables roast perfectly and the sausage warms up nicely.
In a mixing bowl, combine the precooked sausage rounds, sliced bell peppers, and onion. Add the minced garlic, olive oil, Italian seasoning, kosher salt, black pepper, and crushed red pepper flakes (if using). Toss everything together until well coated.
Spread the sausage and vegetable mixture evenly on the sheet pan in a single layer. This helps them roast evenly and develop beautiful caramelization.
Place the sheet pan in the preheated oven and roast for about 20-25 minutes, stirring halfway through for even cooking. You want the veggies to be tender and slightly charred.
Once done, remove from the oven. Garnish with fresh parsley or basil if desired. Serve warm, and enjoy the delightful medley of flavors!