1 tablespoon fresh thyme leaves, plus more for garnish
¼ teaspoon salt
¼ teaspoon ground pepper
1 ½ cups shredded Gruyère cheese
Preheat oven to 375 degrees F. Coat a 9-inch pie pan with cooking spray; set aside.
Heat oil in a large nonstick skillet over medium-high heat; swirl to coat the pan. Add mushrooms; cook, stirring occasionally, until browned and tender, about 8 minutes. Add onion and garlic; cook, stirring often, until softened and tender, about 5 minutes. Add spinach; cook, tossing constantly, until wilted, 1 to 2 minutes. Remove from heat.
Whisk eggs, milk, half-and-half, mustard, thyme, salt and pepper in a medium bowl. Fold in the mushroom mixture and cheese. Spoon into the prepared pie pan. Bake until set and golden brown, about 30 minutes. Let stand for 10 minutes; slice. Garnish with thyme and serve.