Preheat your oven to 425°F. Remove mushroom stems and use a small spoon to gently scoop out the dark gills—this prevents the filling from pooling with excess liquid. Pat caps dry with paper towels to ensure crispy edges when baked.

Heat olive oil in a large skillet over medium heat. Add diced onion and sauté for 3-4 minutes until softened, then add minced garlic and cook 1 minute more until fragrant. This builds the aromatic foundation CIA chefs call the flavor base.

Add fresh spinach to the skillet and cook, stirring constantly, for 2-3 minutes until completely wilted and any excess moisture evaporates. Press spinach against the sides to release all liquid before proceeding to the next step.

Remove the skillet from heat and let cool for 2 minutes. Stir in softened cream cheese, Parmesan cheese, egg, salt, pepper, and nutmeg until completely combined. Fold in half the mozzarella cheese and lemon juice for brightness—this creamy spinach stuffed mushrooms mixture should hold together firmly.

Arrange mushroom caps gill-side-up on a parchment-lined baking sheet. Divide the filling evenly among caps, mounding slightly. Mix remaining mozzarella with breadcrumbs and sprinkle generously over each mushroom—the mixture will turn golden brown.

Bake for 20-25 minutes until the mushroom caps are tender and the tops are golden. The filling should be set but still slightly creamy inside, with no liquid pooling around the edges.

Remove from oven and drizzle with Italian seasoning. Let rest for 2 minutes before serving to allow the filling to set completely.