Melt butter in large nonstick skillet over medium heat. Add breadcrumbs; saute until golden, about 2 minutes. Set aside. Preheat oven to 350°F. Butter two 8×8-inch glass baking dishes. Mix spinach, sour cream, milk, cheese, onion, and nutmeg in large bowl. Season with salt and pepper. Mix in eggs. Divide spinach mixture between dishes. (Can be made 6 hours ahead. Cover; chill.) Top with breadcrumbs. Bake until set, about 30 minutes.