Step 1Roast the potatoes: Heat the oven to 425 degrees. Prick the potatoes all over with a fork. Place on a foil-lined, rimmed baking sheet and roast until a knife easily pierces them, 40 minutes to 1 hour. Remove and set aside until cool enough to handle, then peel and run through a ricer to mash.
Step 2Roast the marrow: Once the potatoes are removed from the oven, place the marrow on a foil-lined, rimmed baking sheet and roast just until the bones begin to brown and the marrow softens, about 15 minutes. Remove and push the marrow out of the bones. Cut the marrow into small pieces and set aside. Discard or save the bones for another use. Reduce the oven temperature to 275 degrees.
Step 3Roast the poblanos: Place the chiles on a rack set over a gas stove-top burner heated over high heat. Roast until the skin on all sides of each chile is charred, about 5 minutes, turning frequently. (If you have an electric or ceramic stove top, roast the chiles in the oven using the broiler setting until charred on all sides.) Wrap each pepper in plastic wrap and set aside until the peppers are cool enough to handle, then peel the skin (the skin should stick to the plastic wrap). Rub the plastic wrap against the skin to loosen and remove it. Stem and seed each pepper, thinly slice. Set aside.
Step 4In a large bowl, gently mix together the masa with the mashed potatoes and garlic oil. Slowly add water until the mixture has the consistency of muffin batter, about 1/3 cup of water. Fold in the bone marrow.
Step 5Pour the mixture into an 8-inch square baking dish. Place the dish in a larger roasting pan. Place the pan into the 275-degree oven, and add boiling water to the roasting pan until it comes 2/3 up the side of the baking dish. Cook until the masa mixture is set, 1- to 1 ½ hours. Remove from heat and set aside until completely cool. Cut the cooked masa into 1-inch cubes.
Step 6Meanwhile, in a large skillet heated over medium-high heat, melt the butter. Add the onions and cook until softened, stirring frequently, about 5 minutes. Stir in the garlic and cook until fragrant, about 1 minute. Stir in the sliced chiles and chard stems and cook until tender, about 10 minutes. Stir in the oregano and cumin, and season with ¾ teaspoon salt and ½ teaspoon pepper, or to taste. Remove from heat and set aside to cool, then stir in the cilantro.
Step 7Heat the oven to 400 degrees. Butter a 13-inch by 9-inch baking dish.
Step 8In a large bowl, gently toss the vegetable mixture with the masa cubes. Stir in the Swiss chard leaves, along with the cheeses.
Step 9In a small bowl, whisk together the eggs and broth. Pour over the large bowl of stuffing and toss gently to moisten. Transfer the stuffing to the buttered dish and cover with foil. Bake for 30 minutes. Uncover and continue to bake until the top is golden-brown, about another 30 minutes.