Bring a large pot of water (5 or 6 quarts) to a boil with 2 teaspoons salt.
Drop spinach into the water and cook 1 minute. Remove with a slotted spoon to a colander and drain. Put the spinach in a clean kitchen towel (careful, it's hot), roll it up and squeeze over the sink to remove as much water as you can. Chop the spinach.
Parboil the pasta in the boiling water until halfway cooked, 7-10 minutes depending on the size and thickness of the pasta. Drain.
Stir together cream, garlic, salt, pepper, chicken, spinach and the cheeses in a large bowl. Add drained pasta and mix together.
Smear the butter over a 1-quart gratin or baking dish. Transfer pasta mixture to the gratin dish.
Bake 12 minutes or so, until the cream is bubbling and edges of the pasta begin to brown. Cool 10 minutes before serving.
This is a perfect dinner for either a hungry couple or 4 people with a salad on the side. Double up the recipe for a larger crowd, using 2 gratins or one 2-quart baking dish.
Leftover roast chicken is ideal for this cheesy baked pasta recipe, or omit the chicken for a vegetarian main dish.
Alternatively, you can roast a large bone-in, skin-on chicken breast half for 25 minutes at 425 degrees. Cool and shred the meat.