1. Cook the ziti according to package directions.
2. Meanwhile, heat the butter and oil in a heavy 12-inch skillet over moderately low heat. Add the onions, and cook, uncovered, for 15 minutes, stirring frequently. Add the garlic, and cook 1 minute longer.
3. Drain the ziti thoroughly in a colander, then toss with the onions in the skillet. Add the cheese, and toss again. Season with the salt and pepper.