Vegetable Shepherds Pie

Name: Vegetable Shepherd's Pie
Ingredients: 2 pounds sweet potatoes, peeled and cut into 2-inch chunks; 1 tablespoon plus 1/2 teaspoon kosher salt; 6 tablespoons unsalted butter; 1 large yellow onion, thinly sliced; 2 parsnips, peeled and sliced 1/4 inch thick; 2 stalks celery, sliced 1/4 inch thick; 2 medium fennel bulbs, cut into a 1/2-inch dice; 2 cups Brussels sprouts, halved; 2 tablespoons chopped fresh flat-leaf parsley leaves; 1/2 teaspoon black pepper; 1 14.5-ounce can vegetable broth; 3 cups fresh spinach or torn Swiss chard leaves (optional); pinch ground nutmeg