Toast coriander seeds and cumin seeds in medium saucepan over medium heat, stirring frequently, until fragrant, about 2 minutes.
Add vinegar, water, sugar, and salt and bring to boil, stirring to dissolve sugar and salt.
Remove saucepan from heat and add onion, carrots, and jalapeño, pressing to submerge vegetables.
Cover and let cool completely, 30 minutes.
(Cooled vegetables can be refrigerated for up to 1 week.)
