Wash, peel, and slice the carrots into thin rounds or skinny sticks so the brine gets in fast.
Pack your sliced carrots into a clean, heat-safe jar.
Leave enough space so the liquid can cover them all the way.
Pour vinegar, water, sugar, salt, chili flakes, mustard seeds, and peppercorns into a small saucepan.
Heat it up over medium, stir until the sugar’s gone, and let it simmer for a couple minutes.
Pour the hot brine over the carrots, making sure they’re totally covered, and seal the jar while it’s still hot.
Let the jar cool off on your counter, then pop it in the fridge for at least a day to build up flavor.