This steamy, biscuit-topped celebration of spring vegetables is like chicken potpie minus the chicken, and it's satisfying in every way.
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.
Melt butter in a medium saucepan over medium heat. Whisk in flour to make a thin paste. Continue to cook, whisking constantly, until paste darkens in color and smells nutty, about 3 minutes. Gradually add broth, whisking to incorporate. Increase heat to medium-high, and cook, stirring often to prevent scorching, until sauce thickens, 3 to 5 minutes. Pour sauceover vegetables in skillet.
Whisk together flour, baking powder, and salt in a medium bowl. Add butter and rub it into the flour with your fingers until mixture resembles crumbs. Gently stir in dill.
Vigorously stir together buttermilk and egg in a small bowl. Stir buttermilk mixture into flour mixture with a few swift strokes until there are no dry pockets in the bowl. The dough will be loose and sticky.
Using wet hands or a large spoon, scoop dough into 6 mounds that you place on top of vegetable mixture in skillet. Sprinkle biscuits with a pinch of salt. Bake until biscuits are brown and cooked all the way through, when a toothpick comes out clean, about 22 minutes.