Cook frozen spinach with a knob of butter until hot.
Meanwhile, fill a large pot with lightly salted water and bring to a rolling boil over high heat. Once the water is boiling, stir in the pasta and return to a boil. Cook uncovered, stirring occasionally, until the pasta has cooked through but is still firm to the bite, about 8 minutes. Drain well in a colander set in the sink.
Combine garlic, olive oil and tomatoes in a large bowl. Stir in the heated spinach. Serve over rotini.