Vegetable Potpies with Sweet Potato Biscuits

Name: Vegetable Potpies with Sweet Potato Biscuits
Ingredients: 1 lb. parsnips; 1 lb. Brussels sprouts; ½ lb. pearl onions; 1 medium head of cauliflower; 1 large celery root; ½ c. extra-virgin olive oil; 20 sage leaves; 1 tsp. chopped sage; 6 sprig thyme; 1 tsp. chopped thyme; Salt and freshly ground pepper; 2½ c. milk; ¼ c. finely chopped yellow onion; 5 sprig flat-leaf parsley; 4 tbsp. unsalted butter; ¼ c. all-purpose flour; ½ c. heavy cream; Pinch of freshly grated nutmeg; Unbaked Sweet Potato Biscuits; 1 large egg