Wash the cucumbers and slice them into thin rounds, then place them in a large bowl with sliced onions.
Sprinkle salt over the vegetables and allow them to rest for about one hour to help remove excess moisture.
Rinse the cucumbers and onions gently under cold water, then drain them well before placing them into clean jars.
Combine vinegar, sugar, mustard seeds, celery seeds, turmeric, and red pepper flakes in a saucepan.
Heat the mixture slowly while stirring until the sugar completely dissolves and the spices become fragrant.
Pour the warm brine over the cucumber slices and onions, ensuring everything is covered.
Cool the jars before refrigerating, then allow the pickles to rest overnight before enjoying their full sweet and tangy flavor.