Slice the zucchini into ¼ inch rounds. Place on a paper towel lined baking sheet and sprinkle with the salt. Let set for 45 minutes to remove moisture from the zucchini.
Preheat oven to 400 degrees.
While the zucchini is setting, make the cheese sauce by adding the cream and cream cheese to a small sauce pan over low heat. Stir until the cream cheese has melted and the mixture is hot.
Add the garlic powder, onion powder, and ground mustard and whisk to combine.
Remove from the heat and stir in 1 ½ cups of cheddar and ¼ cup of Gruyere. Whisk until smooth and creamy.
Pat zucchini dry, wiping off any large amounts of salt, and place half of the zucchini in a greased 8x8 baking dish. Sprinkle with ground pepper.
Spoon half of the cheese sauce over the zucchini and spread to coat.
Place remaining zucchini in the dish, top with the remaining cheese sauce, and sprinkle with the remaining cheddar and Gruyere.
Bake uncovered for 30-35 minutes. Let set for 5 minutes before serving.
Video
Notes
It's important to salt the zucchini and let it set for awhile to remove some of the moisture. If you skip this step, your zucchini gratin will be much more watery. This amount of salt does season the casserole nicely. If you're sensitive to salt, you may want to wipe it off the individual zucchini slices where you can see it. As is, we think this is perfectly salted and flavorful but we admittedly like a fair amount of salt in our foods. You may use a mixture of zucchini and yellow crookneck squash, if preferred.