• Grate the onion. Peel the onion and cut it in half or quarters to grate it. A food processor with the grater attachment is easiest. Alternately, you can slice the onion super thinly. Add the grated onion to a strainer over a bowl and set aside.
• Make half-and-half mixture. In a large sauce pot, combine the half and half with the garlic, thyme, nutmeg and allspice. Bring to a boil and then immediately reduce to a low simmer. Simmer until the the mixture has reduced by about ¼, about 25 minutes. Remove from the stove, strain, and add the salt and pepper. Let it cool until it's warm, or to room temperature.
• Slice potatoes and add them to half and half mixture. Peel the potatoes and slice them as thinly as possible, adding them to the half and half mixture as you go. Make sure the slices aren't stuck together and that all of them become well coated with the mixture. (I use the slicer attachment on my food processor for this, but you can also use a Chef's knife or a mandolin.) Let the potato slices soak for about 30 minutes.

Assembly

• Brush a bit of the melted butter on the bottom and sides of an approximately 9 x 13 x 2-inch baking dish.

  Without wiping them off, add two even layers of the potato slices from the half and half mixture. You should not be able to see the bottom of the dish when you're done. Sprinkle lightly with salt and pepper. DO NOT THROW OUT THE HALF-AND-HALF MIXTURE!

  Squeeze any remaining liquid out of the onions and sprinkle about ⅓ of them evenly on the potatoes, followed by 1 cup of the Gruyère cheese.

  Add ½ of the spinach leaves. (They will not lay flat.)

  Repeat the above layering steps.

  Add another two layers of the potatoes. Do not worry if nothing is laying flat -- at this point, it will look high and rounded, and likely sitting above the edges of the dish. This is how it should be. Set aside the remaining melted butter, onions, cheese, and all of the half and half mixture.
• Compress. Cover the dish tightly with plastic wrap and put a second dish directly on top -- ideally one that's the same size so the bottom of the top dish is pressing directly onto the plastic-covered, top layer of potatoes. Everything will flatten out at this point. Now weigh down the top dish -- canned food is perfect. Fill the top pan with as many cans as will fit, and place this entire set-up in the refrigerator for 1 hour.

  Weighing down the gratin is a way to get any excess liquid out of the potatoes. Remove the the gratin from refrigerator and take off the weights and top dish. Keeping it covered with the plastic, holding it over the sink, slowly pour out any excess liquid from one of the corners. You can hold the palm of one hand down of the top surface of the gratin (on the plastic), to be sure nothing slides around.

  Remove the plastic wrap completely and sprinkle the top with a bit more salt and pepper, and then the remaining onions and cheese.
• Add remaining ingredients. Stir the saved half-and-half mixture to ensure the garlic and herbs are not sitting at the bottom. Pour it into the gratin until it goes about ¾ the way up the sides (it will be about 1½ cups or so). Use a dull knife to move the potatoes a tiny bit along the edges, to be sure it seeps through to the bottom.
• Bake and cool. Drizzle the top with the remaining melted butter, place the pan on a baking sheet and then it in the preheated 375°F oven. Bake for 50 minutes. Then cover the pan loosely with foil and continue to bake for another 25 minutes.

  If the top of the gratin hasn't become golden brown, place it under the broiler briefly, about 30 seconds. Let the gratin sit for at least 45 minutes before cutting and serving.