Rinse the soaked beans in a colander under cold water until the water runs clear, then drain.

Transfer the beans to a large pot and add enough water to cover them by at least two inches. Bring to a boil over high heat, reduce the heat to low, and simmer uncovered for 90-120 minutes, until the beans are fully tender. Use a slotted spoon to skim off any foam that rises to the surface while the beans cook. Drain the beans and allow them to cool until lukewarm.

Transfer the beans to a large bowl. Add the onion, parsley, olive oil, vinegar, lemon juice, salt, and pepper, and toss. Taste and adjust the seasoning, if needed.

Serve at room temperature. 

Leftover fassolia piaz can be stored in an airtight container in the refrigerator for up to 4 days.