Combine ground beef, onions, garlic, soy sauce, sesame oil, sugar, salt, and pepper to taste. Mix gently until it comes together (don't overwork the beef!)
Make a little test patty with the filling and adjust any seasoning if needed.
Slice peppers in half long ways and de-seed.
Cut zucchini into thin rings (~0.5 cm). Place zucchini rings on a paper towel, sprinkle salt, and place another paper towel to draw out any moisture. (Optional: using a ring mold or cap, punch out the center of any larger zucchini slices).
Using an off-set spatula or butter knife, stuff peppers, zucchinis, and perilla/sesame leaves with the ground beef filling.
Coat the stuffed vegetables in flour, making sure to tap off any excess. Then coat with egg wash.
Pan fry in batches, making sure not to overcrowd the pan (~2 minutes each side).