Preheat your oven to 375 degrees F. Lightly spray a 9-inch pie pan with cooking spray and set aside.

In a large nonstick skillet, heat the olive oil over medium-high heat. Add the mushrooms and cook until they are browned and tender, about 8 minutes. Then, add the onions and garlic, cooking until softened, about 5 minutes. Finally, add the spinach and cook until just wilted, 1 to 2 minutes. Remove the skillet from heat.

In a medium bowl, whisk together the eggs, milk, half-and-half, Dijon mustard, thyme, salt, and pepper. Fold in the sautéed mushroom mixture and the shredded Gruyère cheese. Pour this filling into the prepared pie pan.

Bake the quiche in the preheated oven until it is set and the top is golden brown, about 30 minutes. Allow the quiche to cool for 10 minutes before slicing. Garnish with additional thyme leaves and serve.

DEVOUR!