Preheat the oven to 375°F and grease a 9x13-inch baking dish.

Bring a large pot of well-salted water to a boil.

Cook the ziti 1–2 minutes less than the package directions, then drain well.

While the pasta cooks, heat a little olive oil in a large skillet over medium-high heat.

Add the sliced sausage rounds in a single layer and cook until browned and cooked through, letting them sit briefly to develop golden edges.

Push the sausage to one side of the skillet and add the sliced bell peppers.

Cook the peppers until softened but not completely limp.

Pour in the marinara sauce and stir everything together.

Bring the sauce to a gentle simmer, then taste and season with salt and black pepper if needed.

Add the drained ziti to the skillet and fold until the pasta is evenly coated.

Transfer the mixture to the prepared baking dish and spread evenly.

Cover tightly with foil and bake for 20–25 minutes, until bubbling around the edges.

For a slightly crisp top, remove the foil during the last 5 minutes of baking.

Let the ziti rest for 5 minutes before serving.