Place carrots in a saucepan with enough water to cover and cook over medium heat until nearly boiling, 7 to 10 minutes. Immediately drain carrots and set aside to cool.

Divide cooled carrots into two 1-quart glass jars. Alternate layers of onion and jalapeño peppers atop carrots until the jars are full.

Mix the liquid from jalapeño peppers and vinegar in a saucepan; bring the mixture to a rolling boil. Remove from heat and pour the liquid into the jars until full. Seal the jars with lids. Place the jars in the refrigerator until cold, at least 8 hours.