Mushroom Pot Pie with Scallion Drop Biscuits

Name: Mushroom Pot Pie with Scallion Drop Biscuits
Prep time: 30 min
Total time: 30 min
Cuisine: American
Ingredients: 2 tbsp. olive oil, divided, plus more for baking dish; 6 tbsp. (3/4 stick) unsalted butter, divided; 1 3/4 lb. mixed mushrooms, torn or sliced, divided; Kosher salt and freshly ground black pepper; 1 medium yellow onion, chopped; 2 large carrots, chopped; 2 stalks celery, chopped; 3 garlic cloves, chopped; 1/4 c. all-purpose flour; 2 c. vegetable stock; 1 c. whole milk; 1 c. frozen green peas; 2 tbsp. Dijon mustard; 2 tbsp. chopped fresh thyme; 1 tbsp. white wine vinegar; 1 3/4 c. all-purpose flour, spooned and leveled; 1 tbsp. baking powder; 3/4 tsp. kosher salt; 1/2 tsp. freshly ground black pepper; 6 tbsp. (3/4 stick) cold unsalted butter, cut into pieces, plus 2 tablespoons, melted; 1 1/2 oz. Parmesan, grated (about 1/2 cup); 1/2 c. chopped scallions (about 6 medium scallions), plus more for garnish