Whisk together in a bowl until the salt dissolves:
- 2 cups distilled white vinegar
- 2 teaspoons salt
- 1 teaspoon black pepper, or to taste
- 1 teaspoon red pepper flakes, or to taste
Stir in:
- 1 small onion, thinly sliced
- 1 jalapeño pepper, thinly sliced
Use the same day.