Rinse the tomatillos under cold water. Put them in a pot and cover them with water. Bring to a boil, over medium heat. Reduce the heat and simmer uncovered, until their color has changed and they are cooked and soft but not falling apart, about 10 minutes.
Add the tomatillos to a blender along with the chile, the garlic clove and a teaspoon of salt. Puree until smooth. Stir in the chopped cilantro and onion. Taste for salt and add more, if necessary. Pour into a serving bowl and garnish with cheese, if desired.
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