Preheat Oven → Set to 375°F; place rack in middle—grab oven-safe 10-inch skillet.

Sauté Mushrooms → Heat olive oil over medium-high; add mushrooms in one layer—cook 5 minutes undisturbed until golden.

Soften Aromatics → Lower heat; add shallot and garlic—sauté 1 minute until translucent and fragrant.

Wilt Spinach → Pile spinach into skillet off-heat; residual warmth collapses leaves perfectly in 30 seconds.

Whisk Eggs → In large bowl, vigorously beat eggs with milk, nutmeg, salt, and pepper until frothy.

Combine Filling → Spread mushroom-spinach mixture evenly in skillet; sprinkle half the cheese.

Pour Eggs → Gently pour egg mixture over veggies; tilt skillet to distribute evenly.

Cook Stovetop → Return to medium-low; cook 5 minutes until edges set but center jiggles.

Cheese Crown → Sprinkle remaining cheese on top; transfer to oven—bake 10-12 minutes until puffed.

Broil & Serve → Switch to broil 2 minutes for golden top; rest 5 minutes, then slice into wedges.