Heat the oil in a skillet set over medium-high heat.
Place peppers in the skillet and cook, tossing to coat, until peppers are shiny and start to blister, about 5 minutes. Remove from heat.
Cool enough to handle, toss with the cilantro leaves, and transfer whole hot peppers to 1 quart mason jar and press down.
Make the brine. Combine the vinegar, sugar, garlic, peppercorns, salt and pepper flakes in a small saucepan over medium-high heat.
Bring all the way to a boil then remove from heat.
Carefully pour the hot brine over the peppers in mason jar (a funnel makes this easier and safer). Leave 1/2-inch head space between the top of the liquid and the bottom of the jar lid and make sure all peppers are completely submerged.
Screw on 2-piece top and leave on the counter overnight. As the brine cools a vaccuum will form in the jar sealing it for long term storage.
