Whisk together vinegar, water, sugar, salt and cumin. Continue whisking until sugar is dissolved.
Place red onion, jalapenos and garlic in a bowl. Pour vinegar mixture over the top. Let sit at room temperature for 1 hour. Cover and chill for an additional 1 - 2 hours. Serve.
Store onions covered in a glass jar in the refrigerator. Will keep refrigerated for about a week.