Preheat oven: Heat oven to 350°F (175°C). Grease a 12-cup muffin tin with cooking spray or butter.

Prepare vegetables: Heat olive oil in a large skillet over medium heat. Add diced onion and cook for 2-3 minutes until softened.

Cook mushrooms: Add diced mushrooms to the skillet and cook for 4-5 minutes until they release moisture and become golden.

Add garlic and spinach: Stir in minced garlic and cook for 30 seconds. Add chopped spinach and cook just until wilted, about 1 minute. Remove from heat and let cool slightly.

Beat eggs: In a large bowl, whisk together eggs, milk, salt, pepper, and dried herbs until well combined.

Assemble egg cups: Divide the cooked vegetable mixture evenly among the muffin cups. Pour beaten egg mixture over vegetables, filling each cup about 3/4 full.

Add cheese: Sprinkle shredded cheese on top of each egg cup.

Bake: Place in preheated oven and bake for 18-22 minutes, or until eggs are set and lightly golden on top.

Cool and serve: Let cool in the pan for 5 minutes before removing. Garnish with fresh chives and serve warm.