In a bowl, combine the hung curd, chopped coriander leaves, red chilli flakes, and salt. Mix well and set aside.
Heat the oil in a small pan (a tadka pan works perfectly). Add the mustard seeds and chopped curry leaves. Cook until the mustard seeds splutter and the curry leaves become fragrant.
Pour the hot tadka directly into the prepared dahi mixture. Stir thoroughly to incorporate the flavours.
Heat a little oil on a non-stick tawa or frying pan. Place the bread slices on the tawa and toast them until they're slightly golden brown and wonderfully crisp on both sides.
Spread a generous layer of the dahi tadka mixture over each toasted bread slice. Top with the thinly sliced cucumber.
Your Dahi Tadka Cucumber Toast is now ready to enjoy!