Melt 2 Tbsp. butter in a large nonstick skillet over medium heat. Add onion and garlic, and sauté 8 minutes or until tender.
Meanwhile, drain spinach well, pressing between paper towels to remove excess moisture.
Blend cream cheese and flour in a large bowl until smooth. Whisk in eggs, salt, and pepper. Gradually whisk in milk until blended. Add sautéed onions, spinach, and cheese, stirring to blend. Spoon into a lightly greased 11" x 7" baking dish.
Combine breadcrumbs and 3 Tbsp. melted butter in a small bowl; toss well, and sprinkle over casserole.
Bake, uncovered, at 350° for 30 to 35 minutes or until thoroughly heated and breadcrumbs are browned.