Sweet Potato Shepherds Pie

Name: Sweet-Potato Shepherd's Pie
Ingredients: 4 sweet potatoes, peeled and cut into 1-inch pieces (6 cups); 3 tablespoons extra-virgin olive oil; Coarse salt and freshly ground pepper; 1 ½ pounds ground beef (90 percent lean); 8 ounces cremini mushrooms, sliced (3 cups); 1 large onion, chopped (2 cups); ½ cup Pilsner-style beer; 2 teaspoons cornstarch; 2 teaspoons Dijon mustard; 1 ¼ cups low-sodium chicken broth