Bring a 6-qt. saucepan of water to a boil. Cook parsley until bright green and wilted, 30 seconds–1 minute. Using tongs, transfer parsley to a bowl of ice water and let sit for 1 minute. Drain parsley and squeeze dry completely, then coarsely chop and set aside.
Discard water from pan and add butter; melt over medium. Cook garlic, onion, salt, and pepper, stirr