Prepare the steamer by placing a steamer insert into a saucepan and filling with water to just below the bottom of the steamer.

Bring the water to a boil over high heat.

Add the trimmed green beans to the steamer basket, ensuring they are evenly distributed.

Cover the saucepan tightly with a lid.

Steam the green beans until they are tender but still slightly crisp, approximately 2 to 6 minutes.

While the green beans are steaming, prepare the sauce.

In a separate saucepan, melt the butter over medium heat.

Add the minced onion and garlic to the melted butter.

Cook and stir the onion and garlic for 2 minutes, until softened and fragrant.

Add the sesame seeds to the saucepan.

Continue to cook and stir until the sesame seeds are golden brown, about 3 to 5 minutes, being careful not to burn them.

In a small bowl, whisk together the soy sauce, mayonnaise, sriracha hot sauce, and ground allspice until well combined.

Pour the sauce mixture into the saucepan with the onion and sesame seeds.

Stir the sauce mixture to combine it with the onion and sesame seeds.

Remove the steamed green beans from the steamer and add them to the saucepan with the sauce.

Toss the green beans gently to coat them evenly with the sauce.

Season the green beans with salt and ground black pepper to taste.

Serve the green beans immediately while hot.