Preheat oven to 350ºF. Grease a 9-inch square baking dish. In a medium skillet, warm oil over medium-high heat. Add onion and sauté until tender, about 3 minutes. Add garlic and continue to cook, stirring, until soft and fragrant, about 2 minutes longer.
In a large bowl, combine spinach, onion mixture, sour cream, cream cheese, milk, Gruyère, nutmeg, salt and pepper until thoroughly mixed. Stir in egg. Pour into baking dish.
In a small bowl, combine panko and Parmesan. Add butter and mix well. Scatter panko mixture over spinach. Bake, uncovered, until light golden brown and set, about 40 minutes.