Preheat oven to 450°F. Heat oil in a 12-in. ovenproof skillet over medium. Add leeks, carrots, and celery, and cook, stirring often, until mixture is softened but not cooked through, about 10 minutes. Add fennel and salt, and continue to cook until fennel has softened slightly, about 3 minutes. Add artichoke hearts, asparagus, peas, parsley, tarragon, and several grinds of fresh pepper. Cook just until asparagus is bright green, about 2 minutes. Remove from heat.