Rinse the glutinous rice several times until the water runs clear.
Soak the rice in plenty of water for at least 3 hours or overnight. The water should sit at least 5 cm above the rice.
Drain the rice through a sieve. Line a bamboo steamer with greaseproof paper and spread the rice evenly inside.
Steam for 25 to 30 minutes until the rice is soft and translucent.
While the rice is steaming, place the coconut milk, sugar, and salt in a small saucepan. Warm gently over low heat for 2 to 3 minutes until the sugar dissolves. Do not boil. Remove from heat.
Rinse the mung beans and place in a small pot with enough water to cover. Bring to a gentle boil and simmer for 5 minutes until just soft. Drain well and pat dry.
Heat a small frying pan with a little oil. Toast the mung beans for 4 to 5 minutes until golden and crispy. Set aside.
Dice the mango and keep chilled until ready to serve.
When the rice is cooked, transfer it to a bowl while still hot. Pour over about two-thirds of the coconut mixture and mix gently until absorbed.
To serve, spoon coconut rice onto plates. Top with mango and drizzle with the remaining coconut sauce. Sprinkle with crispy mung beans.