Step 1

Set oven racks in middle and upper positions. Preheat oven to 450°F. Place mushrooms and bacon on a rimmed baking sheet. Place on top rack in oven while preheating, and roast, stirring once, until mushrooms are golden brown, 25 to 30 minutes. Place a 10-inch cast-iron skillet on middle rack and heat 15 minutes.

Step 2

Place eggs and garlic in a blender. Process on high until frothy, 45 seconds. With blender running, gradually add milk and 2 tablespoons butter; stop blender. Add flour, cornstarch, and 1/4 teaspoon salt; process 1 minute. Fold in thyme.

Step 3

Carefully add remaining tablespoon butter to heated skillet and swirl to coat. Immediately add batter. Bake until golden brown and puffed, 14 to 16 minutes. Sprinkle with cheese and bake until melted, 3 to 5 minutes.

Step 4

Top with mushroom mixture, scallions, and parsley.