How to Make It
Scrub 1 1/2 pounds medium red potatoes and peel 8 small onions, leaving the stem ends intact. In a large stockpot, combine the potatoes and onions and add about 2 quarts of water, enough to cover the vegetables by 2 inches. Stir in 2 tablespoons kosher salt and bring to a boil over high heat. Partially cover the pot and cook until the potatoes are almost tender when pierced with a fork, about 15 minutes.
Arrange four 1/2-pound whitefish steaks in a single layer on top of the vegetables; don't worry if they aren't covered with water. Partially cover the pot again, lower the heat to moderate and cook until the whitefish flakes easily when tested with a fork, about 10 minutes longer.
Using a slotted spoon, transfer the whitefish to a large, warm platter. Spoon the potatoes and onions around the fish. Sprinkle with 2 tablespoons finely chopped fresh flat-leaf parsley and a generous amount of freshly ground pepper. Serve the fish and vegetables with melted unsalted butter and lemon wedges alongside.