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; cook and stir onion and garlic for 2 minutes. Add sesame seeds; cook and stir until sesame seeds are golden brown, 3 to 5 minutes.
Mix soy sauce, mayonnaise, sriracha hot sauce, and allspice into onion mixture. Add green beans and toss to coat; season with salt and pepper.
