Preheat the oven to 325°F (170°C). Grease a 9 x 13-inch (22 x 33 cm) baking dish with the butter.
Add the chorizo to a large pan over medium-high heat and sauté until cooked through, 5-6 minutes. The chorizo will not change color, but it will give off a lot of red oil and become firm enough to break up with a spatula or wooden spoon.
Transfer the chorizo to a bowl with a slotted spoon, leaving about a tablespoon of oil behind in the pan.
Add the garlic, onion, and poblanos to the pan and sauté until the oil is absorbed and the onions are translucent and red, about 5 minutes.
Add the salt, chorizo, and cream to the pan. Reduce the heat to medium-low and bring to a gentle simmer. Cook for about 10 minutes, stirring frequently, until the cream thickens enough to coat the back of a spoon.
Use a slotted spoon to remove the solids from the cream mixture and set aside. The infused cream should be the consistency of a thin gravy. If not, continue to simmer until that consistency is reached.
Arrange a layer of potato slices in the bottom of the prepared baking dish. Top with some of the chorizo mixture. Repeat with the remaining potatoes and chorizo mixture until the dish is full, finishing with a layer of chorizo and vegetables. Pour the cream sauce over the top and shake the pan slightly so the sauce settles to the bottom of the dish. Cover the pan tightly with foil.
Bake for about 60 minutes, until the potatoes are tender.
Remove the dish from the oven and turn on the broiler. Uncover the pan, and scatter the Oaxaca cheese over the top. Place the pan under the broiler to melt and crisp the cheese, 2–3 minutes. Watch very closely so it doesn’t burn.
Let the scalloped potatoes rest for at least 15 minutes, then garnish with the scallions before serving.