Toss the cucumbers and dill in a large heatproof glass, stainless steel or ceramic bowl.

In a small saucepan, combine vinegar, water, sugar, salt, mustard seeds, peppercorns and red pepper flakes. Bring to a low boil, stirring until sugar and salt dissolve.

Pour the mixture over the cucumbers and stir until well combined.

Let sit at room temperature until cooled.

Place in a large jar and store in refrigerator for up to 2 weeks.