Preheat oven to 400°F. Line a large baking sheet with parchment paper.
Spread the precooked sausage rounds, red and yellow bell pepper strips, and sliced onion on the prepared baking sheet.
Drizzle 1–2 tablespoons olive oil over the sausage and vegetables. Sprinkle the minced garlic, 1 teaspoon Italian seasoning, kosher salt and black pepper to taste, and a dash of crushed red pepper flakes if using.
Toss everything on the pan with tongs or a spatula until evenly coated, then spread into a single, even layer.
Bake for 20 to 25 minutes, stirring once halfway through (about 10–12 minutes). After stirring, spread the ingredients back into an even layer.
Continue baking until the vegetables are tender and slightly browned and the sausage is heated through.
Remove from the oven and serve immediately. Garnish with fresh parsley or basil, if desired.