Slice Chuck Roast and set aside in a bowl. Combine soy sauce, brown sugar, mirin, and sesame oil in a separate bowl and whisk until smooth.
Add grated pear, onion, minced garlic, and ginger. Pour liquid over Chuck Roast and marinate overnight in an airtight container.
Finely slice cucumber and red onion and put in mixing bowl. Bring vinegar, salt, and sugar to a boil. Pour over cucumbers and red onion. Allow sitting overnight.
After the roast is done marinating, sauté on high heat until sugars begin to caramelize.
Place about one ounce of roast in each steamed bun. Top with pickled cucumber and red onions. Garnish with cilantro.