Drain spinach well, pressing between paper towels to remove all excess liquid.
Melt butter in a large skillet over medium heat; add onion and garlic, and sauté 5 minutes or until garlic is lightly browned and onions are tender. Remove from heat, and stir in spinach until well blended; cool.
Whisk together eggs and next 4 ingredients in a large bowl. Whisk in milk and freshly grated Parmesan cheese; stir in spinach mixture, and pour into a lightly greased 8-inch square baking dish.
Bake at 350° for 33 to 35 minutes or until set. Let stand 5 minutes before serving.