Combine the vinegar, salt, sugar, coriander seeds, cumin seeds and 1/2 cup water in a small saucepan and bring to a boil, stirring to dissolve the salt and sugar. Remove the pan from the heat and let cool to room temperature.
Halve, pit and peel the avocados, then cut them into 1/2-inch-thick slices. Place the avocado slices in a wide-mouth pint-size glass jar along with the cilantro sprigs, garlic, jalapeno and lime zest. Pour the cooled brine into the jar and cover. Gently turn the jar upside down and rotate to mix the aromatics, avocado and brine evenly. Refrigerate for at least 2 hours. (The pickled avocado slices will keep, tightly sealed in the refrigerator, for up to 1 week.)
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