Preheat the oven to 375 degrees F (190 degrees C).
Spray a 12-cup silicone muffin pan with cooking spray. Divide spinach and mushrooms evenly among the muffin cups, filling each about 2/3 full. Sprinkle 1 teaspoon mozzarella cheese on top.
Beat eggs and sour cream together until well combined. Season with salt and pepper. Mix in Italian seasoning and garlic salt. Pour into muffin cups until almost full.
Bake in the preheated oven until muffins have risen, tops are lightly browned, and a toothpick inserted into the center comes out clean, 20 to 25 minutes. Loosen muffins with a butter knife and remove from the pan.
