Step 1

Preheat oven to 375°F. Coat a 9-by-13-inch baking dish with cooking spray. Heat oil in a large skillet over medium-high heat. Add mushrooms in an even layer; cook, undisturbed, until starting to brown, 5 to 6 minutes. Stir and continue to cook, undisturbed, until golden brown on the bottom, about 3 minutes. Stir and continue to cook, stirring occasionally, until the mushrooms are browned all over and the liquid has evaporated, about 3 minutes. Add garlic, dry mustard, onion powder and salt; cook, stirring constantly, until fragrant, about 1 minute. Add spinach and cook, stirring constantly, until the spinach wilts, 1 to 2 minutes. Set aside to cool slightly, about 10 minutes.

Step 2

Crack eggs into a large bowl and whisk until completely smooth. Add half-and-half and whisk until combined. Reserve 1/2 cup of the vegetable mixture; scatter the remaining mixture in the prepared baking dish. Sprinkle evenly with 3/4 cup Gruyère. Pour the egg mixture over the top. Sprinkle evenly with the remaining 3/4 cup Gruyère and the reserved 1/2 cup vegetable mixture. Bake until puffed, golden brown and just set, 30 to 35 minutes. Let cool slightly, about 10 minutes. Sprinkle with thyme before serving.