Place a steamer insert into a saucepan and fill with water to just below the bottom of the steamer. Bring water to a boil. Add green beans, cover, and steam until tender, 2 to 6 minutes.

Melt butter in a saucepan over medium heat. Add onion and garlic; cook and stir for 2 minutes. Add sesame seeds; cook and stir until golden brown, 3 to 5 minutes.

Stir soy sauce, mayonnaise, Sriracha, and allspice into onion mixture. Add green beans; toss to coat. Season with salt and black pepper.