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Cut collard greens into approximately 3-inch squares. Heat oil in a large, deep skillet or pot over medium-high heat. Add the serrano chile and garlic, and sautee for a minute or two. Add collard greens, a pinch of sugar, a pinch of salt, and 1/2 cup water; stir to combine. Cover the pan and cook collards for approximately 30-35 minutes, or until sweet and tender. Uncover the pan and simmer for 5-10 minutes more, or until all the liquid has evaporated. Taste and adjust seasoning.
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For the salsa: put the tomatillos, serrano chile, garlic, cilantro, sugar, salt, and a few grinds of black pepper into a a blender or food processor and pulse into a coarse puree. Pour into a bowl, taste, and add more salt if needed.
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To compose a taco: warm tortillas over an open flame or in a dry cast iron skillet over high heat. Top each tortilla with a generous spoonful of collards, a spoonful of tomatillo salsa, and a dollop of creme fraiche. Add avocado slices and a squeeze of lime, if desired.