Heat oven to 475 (245 C) degrees. Bring a large pot of water (5 or 6 quarts) to a boil with 1 tablespoon salt.
Drop 1 pound 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 (carefully, it's hot), roll it up and squeeze over the sink to remove as much water as you can. Chop the spinach.
Parboil ½ pound dried short pasta in the boiling water until halfway cooked, 7-10 minutes depending on the size and thickness of the pasta. Drain.
Stir together 1 ¼ cups heavy cream, 1 tablespoon finely chopped garlic , ½ teaspoon kosher salt, Fresh ground black pepper, 1 cup cooked shredded chicken, ⅓ cup grated Parmesan cheese, ½ cup shredded Fontina, Monterey Jack or Havarti cheese and the drained chopped spinach in a large bowl. Add drained pasta and mix together.
Smear 2 tablespoons butter over a 2-quart gratin or baking dish. Transfer pasta mixture to the dish and spread into an even layer.
Bake 12-15 minutes, until the cream is bubbling and edges of the pasta begin to brown. Cool 10 minutes before serving.
Notes
This recipe makes a hearty dinner for 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.
Other greens: Chopped kale, Swiss chard or other tender greens can replace the spinach. Blanch, squeeze dry and chop them just as you would the spinach.
Make ahead: Assemble through the baking step, then cover and refrigerate up to 2 days or freeze up to 1 month. To bake from frozen, cover the dish with foil for the first half of the bake time, then uncover, and add 15 minutes to the total.