Mix the oil and water (don't worry if it doesn't combine- it's oil and water). Add to the flour and mix until well blended. It will seem pretty wet and oily. If you have time: refrigerate this for 15-30 minutes before assembling the manju so the dough is firmer and easier to work with (rather than mush that sticks to your hands).
Divide the dough into 24 pieces.
Using 1 piece of dough at a time, flatten it in your palm.
Place 1 heaping tsp tsubushian or koshian in the center. Wrap the dough (don’t worry if it breaks or isn’t smooth- you can patch it together) around the red bean paste until you can only see the dough.
Flatten slightly and place on a baking sheet (they don’t spread much, so you can place them close together). Brush with egg wash. If you’d like, sprinkle with sesame seeds. Refrigerate the pans while you wait for the oven to preheat.
Bake at 400F for 30-35 minutes, or until golden brown. Don't freak out if they're a little cracked and crumbly; they're still delicious. Cool slightly, then move to a baking rack to cool completely. The manju will keep at room temperature for 1 week.
From my Aunty Lily.
Keep the dough as cold as you can! Using cold oil (I try to measure it out and put it in the fridge for 10-15 minutes before using) and refrigerating the dough once it’s mixed makes this dough firmer and easier to use when you’re trying to encase the red bean paste.
You can ignore the egg wash if you're keeping these vegan.