2 teaspoons olive oil
Coarse salt and ground pepper
1 pound zucchini, cut diagonally into 3/4-inch slices
1 tablespoon onion, minced
1 tablespoon white-wine vinegar
1 tablespoon chicken broth
1/2 cup (1 stick) chilled butter, cut into 8 tablespoons
1 tablespoon chopped fresh flat-leaf parsley
Preheat oven to 500 degrees. Pat halibut dry; rub with 1 teaspoon oil, and season with salt and pepper. Arrange fillets several inches apart on a 10-by-15-inch rimmed baking sheet.
In a small bowl, toss zucchini with remaining teaspoon oil; season with salt and pepper. Arrange zucchini around fish on sheet. Roast in oven, rotating sheet once, until zucchini is cooked through and fish flakes easily, 10 to 12 minutes.
While fish roasts, combine onion, vinegar, chicken broth, and 1/4 cup water in a small saucepan; bring to a boil. Reduce heat to medium; simmer until almost all liquid has evaporated, 5 to 7 minutes. Reduce heat to low; whisk in butter, 2 tablespoons at a time, letting each addition melt before adding next. Stir in parsley; season sauce with pepper. Spoon butter sauce over halibut and zucchini; serve immediately.