Place mung bean flour in a medium bowl; add 1 cup of water and stir to mix well.
Bring 1 cup of water to a boil, add mung bean slurry and cook, stirring, until translucent.
Transfer mung bean paste into a bowl. Cover well and refrigerate overnight.
To make sauce, heat cooking oil in a small saucepan. Add ginger, garlic and green onion and cook until fragrant, about 2 minutes.
Add spicy bean paste, fermented black beans, soy sauce, black vinegar and sugar. Stir to combine and cook for 5 minutes.
When mung bean jelly is set, remove from bowl and cut into ¼” x ¼” x 4” strips. Place on a serving plate and top with spicy sauce. Garnish with green onions and chili oil.