Source: allrecipes.com, Chef John

Ingredients 

3/4 cup water
3/4 cup distilled white vinegar
3 tablespoons white sugar
1 tablespoon kosher salt
1 clove garlic, crushed
1/2 teaspoon oregano
10 large jalapeno peppers, sliced into rings

Directions

Combine water, vinegar, sugar, kosher salt, garlic, and oregano in a saucepan over high heat. Bring mixture to a boil, stir in jalapeno peppers and remove from heat. Let mixture cool for 10 minutes.
Pack peppers into jars using tongs, cover with vinegar mixture, cover, and refrigerate until needed.