Trim the mushrooms into serving pieces and dredge them in glutinous rice flour. Set aside.
In a separate bowl, combine Glutinous rice flour, gochugaru, chickpea flour, salt, and water and mix until it turns into a smooth paste (similar to a thick soup).
Dip the mushrooms in the batter and fry them in oil over medium heat until crisp and golden brown. Drain excess oil on paper towels and arrange them on a serving plate.
Melt some butter in a small pan. Add in chopped garlic and sliced leeks and fry until the leeks are crispy.
Drizzle the butter mixture over the fried mushrooms and serve.