Step 1

Preheat oven to 400 degrees.

Step 2

In a large saucepan, bring 2 inches of water to a boil. Add the spinach, a handful at a time, waiting for it to wilt before adding more. When all the spinach is wilted, drain in a colander. Using a large spoon, press out as much as the water as possible.

Step 3

In the saucepan, melt 2 tablespoons butter over medium heat. Add the mushrooms, onion and garlic and cook until softened and until some of the mushroom liquid cooks away, stirring often. Reduce the temperature to medium-low and stir in the flour.  Cook for an additional minute stirring constantly. Slowly whisk in the milk and simmer until thickened, whisking often. Stir in the salt, pepper, nutmeg, cheese and spinach.  Spoon into baking dish. Sprinkle with panko crumbs. Bake for 20 minutes, until golden brown.