4 tablespoons extra-virgin olive oil, divided
4 (6-oz.) skinless halibut fillets
3/4 teaspoon black pepper
1 teaspoon kosher salt, divided
8 ounces fresh asparagus, trimmed and cut into 1-in. pieces (2 cups)
1 cup thinly sliced leek (from 1 medium [4 oz.] leek)
1 cup sliced yellow bell pepper (from 1 medium [8 oz.] bell pepper)
1 teaspoon lemon zest (from 1 small lemon)
1 cup frozen baby sweet peas, thawed
2 tablespoons unsalted butter
1 tablespoon chopped fresh flat-leaf parsley
Heat 2 tablespoons of the oil in a large nonstick skillet over medium-high. Pat halibut fillets dry; sprinkle evenly on both sides with black pepper and 1/2 teaspoon of the salt. Add fillets to skillet; cook, undisturbed, until deeply browned and a thermometer inserted in thickest portion of fillets registers 140°F, 4 to 5 minutes per side. Transfer to a plate; cover to keep warm.
Wipe skillet clean; add 1 tablespoon of the oil. Add asparagus and leek; cook over medium-high, stirring often, until asparagus begins to soften and leek is tender, about 3 minutes. Add bell pepper and lemon zest; cook, stirring often, until bell pepper softens, 1 to 2 minutes. Add peas, butter, and remaining 1/2 teaspoon salt; cook, stirring often, until butter melts and peas are warmed through, about 1 minute.
Spoon about 2/3 cup vegetables onto each of 4 plates; top with 1 fillet. Drizzle evenly with remaining 1 tablespoon oil; sprinkle with parsley. Serve hot.