Set oven racks in middle and upper positions; heat oven to 450°F. While oven is heating, arrange mushrooms and bacon on rimmed baking sheet and place on upper oven rack. Roast, stirring once, until mushrooms are golden brown, 25 to 30 min.
Place medium cast-iron skillet on middle rack and heat 15 min.
In blender, combine eggs and garlic and blend on high until frothy, 45 sec. With blender running, gradually add milk and 2 tbsp. butter. Add flour, cornstarch, and salt; blend 1 min. Fold in thyme.
Carefully add remaining tbsp. butter to heated skillet and swirl to coat. Immediately add batter. Bake until golden brown and puffed, 14 to 16 min. Sprinkle with cheese and bake until melted, 3 to 5 min.
Top with mushroom mixture, scallion, and parsley.
