Step 1

Preheat oven to 325 degrees F. Heat oil in a large nonstick skillet over medium-high heat. Add mushrooms in an even layer; cook, undisturbed, until browned on the bottom, about 4 minutes. Stir and continue to cook, stirring occasionally, until browned all over, about 5 minutes. Add onion; cook, stirring occasionally, until beginning to soften, about 4 minutes. Stir in garlic and thyme; cook, stirring, until fragrant, about 2 minutes. Add spinach; cook, stirring constantly, until just wilted, about 2 minutes. Remove from heat.

Step 2

Whisk eggs, milk, Dijon, salt and pepper in a large bowl. Stir in cheese and the mushroom mixture. Coat a standard 12-cup muffin tin with cooking spray. Divide the mixture among the prepared muffin cups. Bake, uncovered, until puffed and set, about 30 minutes. Remove from the pan and serve immediately.