Ingredients
1/2 pounds golf ball-size baby potatoes
Kosher salt and freshly ground pepper
1 pound cipollini onions
1/4 cup white-wine vinegar
2 teaspoons Dijon mustard
1/2 cup plus 1 tablespoon extra-virgin olive oil, and more for brushing
1 bunch fresh oregano
1 cup mixed olives, such as Kalamata, Castelvetrano, and Cerignola
1 1/2 pounds halibut steaks or skin-on fillets (about 1 1/2 inches thick), preferably Alaskan
1 ciabatta or other rustic loaf, split
Peppery greens, such as arugula and dandelion, for serving
Preparation
In a pot, cover potatoes with 2 inches water; generously season with salt. Bring to a boil over high heat; cook 8 minutes. Add onions; continue boiling until potatoes are easily pierced with the tip of a knife and onions are crisp-tender, 4 to 5 minutes more. Drain; let cool slightly. When cool enough to handle, peel onions.
Preheat grill to medium-high. In a wide, shallow dish, whisk together vinegar, mustard, and 1/2 cup oil. Chop enough oregano to yield 3 tablespoons (from 2 to 3 sprigs) and stir into vinaigrette with olives; season with salt and pepper. In another bowl, toss potatoes and onions with remaining 1 tablespoon oil; season with salt and pepper. Brush fish and remaining oregano sprigs with oil; season with salt and pepper.
Brush grates with oil, add vegetables, and grill, turning a few times, 6 minutes. Add fish to grill and top with oregano. Cook fish and vegetables, turning fish once halfway through, until fish is just cooked and vegetables are charred in places and tender, 8 to 10 minutes more. Transfer to dish with vinaigrette, turning to evenly coat. Let stand, turning a few times, until no longer hot but still warm, 10 minutes. Meanwhile, grill split side of bread until charred in places, 1 to 2 minutes; slice. Serve fish, vegetables, and vinaigrette over greens, with grilled bread.
