Place jalapeños, onion, and garlic in a large glass jar with a tight-fitting lid.
Combine 2 cups water and salt in a saucepan; bring to a boil over high, stirring until salt dissolves. Cool to room temperature.
Pour salted water over jalapeño mixture. Cover with lid; set on counter in a cool place away from direct sunlight for 2 to 3 days, stirring once a day.
Process mixture in a blender until smooth. Serve immediately, or refrigerate in an airtight container up to 2 weeks.
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