Bring all ingredients except jalapeños to a boil in a medium sized pot (4-5 cloves smashed garlic, 2 cups white vinegar, 2 cups water, 1/2 cup sugar, 2 tablespoons kosher salt, 2 teaspoons dried oregano).
Add jalapeño slices to the pot, stir together, and remove from heat. Let sit for at least 10 minutes.
Use tongs or a slotted spoon to transfer the jalapeños to a jar. Once all the peppers are in there (you can smush them in quite a bit to make them fit), pour enough brine from the pot in the jar to fill it up. Discard any remaining brine.
Allow the pickled jalapeños to cool in your refrigerator for about an hour before consuming. Store in your fridge for up to 2 months for optimal freshness.
To make these less spicy, you can use older jalapeños that aren't as fresh, and try to remove as many seeds as possible by rinsing them with water and using tongs to transfer them to and from the pot. They will mellow out a bit the longer they sit in the fridge as well, and you can give them a rinse in a colander after slicing before adding to the brine to help take it down a notch. I personally used a mild variety of jalapeños I grew in my garden. Homegrown jalapeños become spicier the longer they sit on the plant, so pick them young for a more mild taste.
Ingredient substitutions: I don't recommend substituting the white sugar or white vinegar with another sweetener or vinegar. But you can use garlic powder or omit the garlic if you prefer, and use another herb like thyme or omit the oregano entirely.
For safety, I recommend wearing gloves when slicing the jalapeños.
The provided nutrition information does not account for all the brine, as only a small amount of the brine is absorbed into the jalapeños. It is my best estimate.