Preheat oven to 375 degrees F. Grease a 2-quart casserole dish. Cook the pasta until just al dente (do not overcook). Drain and set aside.
While the pasta is cooking, heat the olive oil in a large skillet over medium-high heat. Add the onions and peppers and sauté for about 3 minutes, then add the sliced mushrooms and cook for another few minutes until the vegetables are tender. Remove from heat.
In a large bowl, mix together the cooked vegetables, cooked pasta, pepperoni, marinara or spaghetti sauce, black olives, and half of the shredded cheese. Pour this mixture into the casserole dish and top with the remaining cheese. Bake in the oven for 15-18 minutes until heated through and the cheese browns on top.
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