Preheat the oven to 375°. Rub the cut sides of a garlic clove on the bottom and sides of a 9 x 13-inch casserole or 6 individual oval gratin dishes (10 1/2-inch)
The gratin is easier to assemble by first prepping all of the ingredients. Peel and slice the potatoes, placing the potatoes immediately into large bowl of ice water as you work to prevent browning. Prepare the peeled, seeded green chiles by slicing lengthwise and opening up flat into a single layer. Grate the cheese and place the cream in a large saucepan on stovetop over a medium heat and bring to a simmer.
Drain and pat dry the potatoes in a dishtowel then add into the saucepan of simmering cream for 5 minutes. Using a slotted spoon, remove the potato slices placing half in the baking dish or individual baking dishes to create the first layer. Sprinkle with a pinch of salt and white pepper. If using the green chiles, arrange flat in a single layer over the potatoes. Adjust the quantity of chiles to suit your individual taste, about 2-3 chiles for the large gratin, about 1/2 a chile per individual gratin.
Cover the chiles with the remaining potatoes, pressing down to smooth evenly. Pour the remaining cream from the saucepan over the potatoes, distributing evenly if making individual gratins, top with the grated Gruyere cheese and another sprinkle of salt and white pepper.
Place the gratin(s) on a baking sheet and bake in a 375° oven for about 45 minutes, or until the potatoes are tender and the top is browned and bubbly. Remove from the oven and allow to rest about 10 minutes before serving. Sprinkle the top the minced fresh chives and serve. The gratins may be made a day or two ahead, store covered in the refrigerator. Reheat in covered with foil in a 350° oven for about 30 minutes.