First Step: Preheat and prepare
Preheat your oven to 425°F (220°C). Line a large baking sheet with parchment paper or lightly grease it with cooking spray or olive oil to prevent sticking. Gather all ingredients, slicing the red onion, zucchini or yellow squash, and baby peppers. Quarter mushrooms if using.

Second Step: Arrange sausage pieces
Cut the fully cooked Italian chicken sausages into four pieces each and evenly arrange them on the prepared baking sheet. If adjusting for vegan or gluten-free diets, substitute with the preferred plant-based or gluten-free sausages.

Third Step: Add vegetables
Distribute the sliced onion, zucchini, baby peppers, and mushrooms evenly around the sausage on the baking sheet, ensuring a good mix for uniform roasting.

Fourth Step: Season and toss
In a small bowl, whisk together the extra-virgin olive oil, minced garlic, Italian seasoning, kosher salt, and red pepper flakes if using. Drizzle this mixture over the sausage and vegetables on the sheet pan. Toss gently but thoroughly to coat all pieces evenly in the flavorful oil and seasoning.

Fifth Step: Roast in the oven
Place the sheet pan in the preheated oven and roast for 20 to 25 minutes. About halfway through cooking, stir and flip the sausage and vegetables to promote even caramelization and cooking on all sides. If adding potatoes or root vegetables, parboil beforehand for a tender interior.

Sixth Step: Check for doneness
Check that the Italian sausage pieces reach an internal temperature of at least 165°F (74°C) using a meat thermometer. Ensure that the vegetables are tender and slightly caramelized. For vegan alternatives, roast till heated through and slightly crisp.

Final Step: Serve and garnish
Remove the sheet pan from the oven and let rest for a few minutes. Generously sprinkle freshly grated Parmesan cheese over the top and garnish with fresh parsley or cilantro if desired. Serve immediately as a standalone meal or alongside your favorite grain like rice, quinoa, or pasta for a complete dinner.