Combine soy sauce, rice vinegar, fish sauce and sugar in a small bowl, stirring to dissolve sugar. Set sauce aside.
Trim stem ends of green beans. Heat 1 Tbsp. oil in a large skillet over medium-high. Cook beans, tossing occasionally, until blistered in spots and crisp-tender, about 5 minutes. Transfer to a plate.
Add remaining 1 Tbsp. oil to same skillet and cook shiitake mushrooms, tossing occasionally, until browned and tender, about 5 minutes. Add shallot, garlic, and ginger,and cook, tossing occasionally, just until aromatics have softened, about 3 minutes. Add the chili sauce and reserved “XO” sauce and cook for another minute or 2. Add the sesame oil and reserved green beans and toss well to combine. Season with a dash of kosher salt if needed.
Recipe by Allons-Eat! at https://www.allonseat.com/green-beans-with-mushroom-xo-sauce/