How to Make It
Preheat the oven to 350°. Butter a shallow 1 1/2-quart gratin dish.
Using your hands, squeeze as much moisture as possible from the defrosted spinach, then add it to a large bowl, along with the artichoke hearts, Parmesan cheese, mayonnaise and mustard. Stir until well combined, then season with salt and pepper and set aside.
In a large pot of boiling well-salted water, cook the pasta until al dente. Reserve 1 cup of the cooking water, then drain well. Add the pasta to the spinach mixture and stir to combine, adding some of the cooking water as needed to loosen the mixture.
Pour into the buttered gratin dish, cover with aluminum foil and bake until hot and bubbling, 15 to 25 minutes.