To make this dish in advance, wait until just before serving to combine the beans and onions. Both can be kept refrigerated for a couple ofdays in advance. Mulberry syrup is also delicious here in place of the pomegranate molasses. Find them both here.
In a small bowl, whisk the pomegranate molasses, lemon juice, olive oil, salt, pepper, and garlic until well combined. Add the sliced onion and stir to coat entirely with the vinaigrette. Cover and marinate for 1hour.
In a medium saucepan, bring plenty of water with a tablespoon or so of salt to a boil. Add the beans and remove the pan from the heat. Allow the beans to cook in the hot water until they are bright and crisp, about 10 minutes. Drain.
In a serving dish, toss the beans with the onions and vinaigrette. Taste, and adjust seasonings as desired. Serve immediately.