Char tomatillos, onion slices, garlic and chilies 4 to 5 minutes in a cast-iron skillet over high heat (or under broiler), turning to char multiple sides. You might have to do this step in batches, depending on pan size.

Put charred ingredients in a blender with cilantro and purée. Heat 1½ tablespoons oil in a large pan (you can use the castiron skillet from the charring step) over medium heat, add sauce and fry for 5 minutes. Add vegetable stock and cream and simmer, covered, for 30 minutes. Taste and adjust seasoning with salt and pepper as desired.

Preheat oven to 350°. Place sweet potato cubes in a single layer on a parchment- paper-lined baking pan. Lightly spray cubes with cooking oil spray. Roast sweet potato cubes for 10 to 15 minutes, until barely tender. (Keep oven on.)

Wash chard well. Cut stems to bottom of leaves and slice into ½-inch slices. (If the ribs are thick, slice them out of the leaves). Slice leaves crosswise into 1-inch strips. Heat a large sauté pan over medium-high heat with remaining 1½ tablespoons oil. Add chard stems and onions and cook 5 minutes. Add chard leaves and cook until just wilted. In a bowl, add chard and roasted sweet potato cubes; season with salt and pepper. (If chard has liquid at the bottom of pan, place in a strainer before combining with sweet potatoes.)

Heat a thin layer of oil in a small frying pan over high heat and briefly fry both sides of each tortilla to soften. Drain on a paper towel. Place ½ cup vegetable mixture in center of 12 tortillas and top each with 2 tablespoons cheese. Fold tortilla flaps over to encase filling. Place seam side down in a large ovenproof baking dish.

Top with tomatillo sauce and remaining cheese. Bake 25 to 30 minutes, until heated through and cheese has melted. Garnish with cilantro and crema.