Place cucumber slices in a bowl that just fits them.
Heat water, vinegar, salt, pepper flakes, garlic, black peppercorns, mustard seeds, and dill in a skillet over high heat until salt has dissolved and mixture is simmering.
Immediately pour brine over cucumbers.
Cover bowl with a paper towel pressed directly against the surface of the liquid to keep cucumbers submerged.
Wait at least 30 minutes before serving (do not drain brine).
Pickles can be stored in their brine in a sealed container in the refrigerator for up to a month.
