While you cook the pasta, chop the onion and mince the garlic. Heat the olive oil, and add the onion and garlic. Cook until they are slightly soft, then lower the heat and add the spinach. Cover the pan to help unthaw the spinach, stirring once in a while. When the spinach is heated through, toss with the pasta.
This is very inexpensive and satisfying. I took it with me for lunch in high school all the time, and it tastes good at any temperature.