Combine the vinegar, 1 cup water, the sugar, dill, garlic, jalapeno, pickling spice, and salt in a medium nonreactive pot and bring to a boil over high heat.
Add the onions and cook until a knife inserted into the center meets a little resistance, 4 to 5 minutes.
Transfer the mixture to a nonreactive bowl and cover with plastic wrap, or to a large glass jar with a tight-fitting lid, and refrigerate for at least 24 hours before serving.
The onions will keep for 1 week in the refrigerator.